DynamicJsonDeserializer.cs

// ReSharper disable CheckNamespace
namespace RestSharp.Deserializers
// ReSharper restore CheckNamespace
{
    public class DynamicJsonDeserializer : IDeserializer
    {
        public string RootElement { get; set; }
        public string Namespace { get; set; }
        public string DateFormat { get; set; }

        public T Deserialize<T>(RestResponse response) where T : new()
        {
            return JsonConvert.DeserializeObject<dynamic>(response.Content);
        }
    }
}

ExampleUsage.cs

// Override default RestSharp JSON deserializer
client = new RestClient();
client.AddHandler("application/json", new DynamicJsonDeserializer());

var response = client.Execute<dynamic>(new RestRequest("http://dummy/users/42"));

// Data returned as dynamic object!
dynamic user = response.Data.User;

// Remember properties are actually Json.NET JObjects so you have to call .Value to retrieve their string contents.
string firstName = user.FirstName.Value;
string lastName = user.LastName.Value;