FWIW, if you have an ApiController, instead of a Controller, here is how you can do it:

public class InfoController : ApiController
{
    // Without custom header
    public IHttpActionResult MyMethod(..)
    {
        var myObject= GetMyResult();
        return Ok(myObject);
    }

    // With custom header
    public IHttpActionResult MyMethod(..)
    {
        var myObject = GetMyResult();

        // inspired from https://learn.microsoft.com/en-us/aspnet/web-api/overview/formats-and-model-binding/content-negotiation#how-content-negotiation-works
        var negotiator = Configuration.Services.GetContentNegotiator();
        var result = negotiator.Negotiate(typeof(TypeOfMyObject), Request, Configuration.Formatters);
        var msg = new HttpResponseMessage(HttpStatusCode.OK)
        {
            Content = new ObjectContent<TypeOfMyObject>(myObject, result.Formatter,result.MediaType.MediaType)
        };

        msg.Headers.Add("MyCustomHeader", "MyCustomHeaderValue");
        return ResponseMessage(msg);
    }
}