gistfile1.cs

    [TestFixture]
    public class Example
    {
        public class Bus
        {
            public void SubscribeAsync<T>(string subscriptionId, Func<T, Task> onMessage) { }
        }

        public static MethodInfo GetMethod<T>(Expression<Action<T>> methodSelector)
        {
            var body = (MethodCallExpression)methodSelector.Body;
            return body.Method;
        }

        [Test]
        public void Should()
        {
            var bus = new Bus();

            MethodInfo info = bus.GetType().GetMethods()
                .Where(m => m.Name == "SubscribeAsync")
                .Select(m => new { Method = m, Params = m.GetParameters() })
                .Single(m => m.Params.Length == 2
                    && m.Params[0].ParameterType == typeof(string)
                //&& m.Params[1].ParameterType.GetGenericTypeDefinition() == typeof(Func<>) //<-- not sure how to pull this off?
                    ).Method;
            Console.WriteLine(info.Name);

            var tree = GetMethod<Bus>(b => b.SubscribeAsync((string)null, (Func<int, Task>)null));
            Console.WriteLine(tree.Name);
        }
    }