Basic API wrapper around a Restful service - follow-up
As @t3chb0t says, the implementation is clean enough already.

However, if you wanted to go one step further, I would inject the implementation of IRestSharp into the TopmanClient, so that you can then test the behaviour of your default implementation when an API error occurs. For instance, if the API returns a 404 Not Found, how will your client behave? How about if a deserialisation error occurs?

For double bonus points, you could run the test once passing in a mock IRestClient; then run the same test again passing in a real RestClient - you now have a test of your client and an integration test for the API (although make sure you are pointing at a non-production API, or a test account, or a sandbox). Whether or not you include those in your automated builds or automated deployments, I'll leave up to you.