MockingAsyncTask.cs

    [TestFixture]
    public class MockingAsyncTask
    {
        [Test]
        public async void can_mock_interface_methods_with_task_return()
        {
            var mock = NSubstitute.Substitute.For<ILongRunningOperation>();
            var sut = new SystemUnderTest {Operaiton = mock};

            await sut.Execute();

            mock.DoSomething().ReceivedCalls();
        }

        public class SystemUnderTest
        {
            public ILongRunningOperation Operaiton { get; set; }

            public async Task Execute()
            {
                await Operaiton.DoSomething();
            }
        }

        public interface ILongRunningOperation
        {
            Task DoSomething();
        }
    }

mock.Received().DoSomething(); is what you should be using to assert that a specific method was called