Equivalent of mock verifySet when using FluentAssertion

Question

I'm currently using FluentAssertion and wondering what should be the equivalent of mock verifySet?

[TestMethod]
public void Add()
{
  // Arrange
  var key = _fixture.Create<string>();
  var value = new object();

  var session = new Mock<ISessionState>();
  var states = new HttpSessionState(session.Object);

  // Act
  states.Add(key, value);

  // Assert
  session.VerifySet(s => s[key] = value, Times.Once);
 }