In large pot of boiling salted water, cook pasta according to package instructions; drain well and set aside.
Melt butter in medium saucepan over medium heat. Whisk in flour until lightly browned, about 1 minute. Gradually whisk in milk and heavy cream, and cook, whisking constantly, until incorporated, 1-2 minutes. Stir in mozzarella, a handful at a time, until smooth, about 1-2 minutes; season with salt and pepper, to taste. Remove from heat; add cheese sauce to pasta and stir.
Serve immediately, topped with tomatoes and basil, if desired.