NancyRazorViewEngine.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Abstractions;
using Microsoft.AspNetCore.Mvc.ModelBinding;
using Microsoft.AspNetCore.Mvc.Razor;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.AspNetCore.Routing;
using Microsoft.Extensions.DependencyInjection;
using Nancy;
using Nancy.Extensions;
using Nancy.Responses;
using Nancy.ViewEngines;

namespace www.SupportClasses
{
    public class NancyRazorViewEngine : IViewEngine
    {
        private readonly IRazorViewEngine _razorViewEngine;
        private readonly ITempDataProvider _tempDataProvider;
        private readonly IServiceProvider _serviceProvider;

        public NancyRazorViewEngine(IServiceProvider dotNetServices)
        {
            var scope = dotNetServices.CreateScope();
            _razorViewEngine = scope.ServiceProvider.GetService<IRazorViewEngine>();
            _tempDataProvider = scope.ServiceProvider.GetService<ITempDataProvider>();
            _serviceProvider = scope.ServiceProvider;
        }

        public void Initialize(ViewEngineStartupContext viewEngineStartupContext)
        {
        }

        public Response RenderView(ViewLocationResult viewLocationResult, dynamic model, IRenderContext renderContext)
        {
            var viewName = viewLocationResult.Location + "/" + viewLocationResult.Name;

            var httpContext = new DefaultHttpContext { RequestServices = _serviceProvider };
            var actionContext = new ActionContext(httpContext, new RouteData(), new ActionDescriptor());

            using (var sw = new StringWriter())
            {
                var viewResult = _razorViewEngine.FindView(actionContext, viewName, false);

                if (viewResult.View == null)
                {
                    throw new ArgumentNullException($"{viewName} does not match any available view");
                }

                var viewDictionary = new ViewDataDictionary(new EmptyModelMetadataProvider(), new ModelStateDictionary())
                {
                    Model = model
                };

                var viewContext = new ViewContext(
                    actionContext,
                    viewResult.View,
                    viewDictionary,
                    new TempDataDictionary(actionContext.HttpContext, _tempDataProvider),
                    sw,
                    new HtmlHelperOptions()
                );

                viewResult.View.RenderAsync(viewContext).Wait();
                var response = new HtmlResponse
                {
                    Contents = stream =>
                    {
                        using (var writer = new StreamWriter(stream))
                        {
                            writer.Write(sw.ToString());
                            writer.Flush();
                        }
                    }
                };
                return response;
            }
        }

        public IEnumerable<string> Extensions => new[] { "cshtml" };
    }

    public class NancyViewLocationExpander : IViewLocationExpander
    {
        public void PopulateValues(ViewLocationExpanderContext context)
        {

        }

        public IEnumerable<string> ExpandViewLocations(ViewLocationExpanderContext context, IEnumerable<string> viewLocations)
        {
            yield return "/{0}.cshtml";
        }
    }
}

Startup.cs

using System.Diagnostics;
using System.IO;
using System.Reflection;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Hosting.Internal;
using Microsoft.AspNetCore.Mvc.Razor;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.FileProviders;
using Microsoft.Extensions.ObjectPool;
using Microsoft.Extensions.PlatformAbstractions;
using Nancy.Owin;
using www.SupportClasses;

namespace www
{
    public class Startup
    {
        private readonly IConfiguration _config;
        private IServiceCollection _services;
        public Startup(IConfiguration configuration)
        {
            _config = configuration;
        }

        public void ConfigureServices(IServiceCollection services)
        {
            var applicationEnvironment = PlatformServices.Default.Application;
            services.AddSingleton(applicationEnvironment);

            var appDirectory = Directory.GetCurrentDirectory();

            var environment = new HostingEnvironment
            {
                ApplicationName = Assembly.GetEntryAssembly().GetName().Name
            };
            services.AddSingleton<IHostingEnvironment>(environment);

            services.Configure<RazorViewEngineOptions>(options =>
            {
                options.FileProviders.Clear();
                options.FileProviders.Add(new PhysicalFileProvider(appDirectory));

                options.ViewLocationExpanders.Clear();
                options.ViewLocationExpanders.Add(new NancyViewLocationExpander());
            });

            services.AddMvc();
            _services = services;
        }

        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            app
                .UseOwin(owin => owin
                    .UseNancy(nancy =>
                    {                      
                        nancy.Bootstrapper = new Bootstrapper(_config, app.ApplicationServices);                      
                    })
                );
        }
    }
}

Does it work on 3.1.1?