Program.cs

using System.Data.Entity;
using System.Data.Entity.ModelConfiguration.Conventions;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            using (var ctx = new Ctx())
            {
                var contaAReceber = new ContaAReceber();
                contaAReceber.Valor = 100;
                contaAReceber.NFSe = new NotaFiscalEletronicaServico();
                contaAReceber.NFSe.Valor = 200;

                ctx.ContaARecebers.Add(contaAReceber);
                ctx.SaveChanges();
            }
        }
    }

    public class Ctx : DbContext
    {
        public DbSet<ContaAReceber> ContaARecebers { get; set; }
        public DbSet<NotaFiscalEletronicaServico> NotaFiscalEletronicaServicos { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
            base.OnModelCreating(modelBuilder);
        }
    }

    public class ContaAReceber
    {
        public int Id { get; set; }
        public decimal Valor { get; set; }
        public NotaFiscalEletronicaServico NFSe { get; set; }
    }

    public class NotaFiscalEletronicaServico
    {
        public int Id { get; set; }
        public decimal Valor { get; set; }
        public ContaAReceber ContaAReceber { get; set; }
    }
}