Cut about 1/2 cup leftover cooked spaghetti noodles into 1-inch lengths. Optional: crisp noodles slightly in skillet with 1/2 teaspoon olive oil.
In a medium bowl, whisk together 4 large eggs, 3 tablespoons milk, and 1/4 teaspoon salt. Stir in 1/3 cup grated low-moisture mozzarella cheese and 1/4 cup (about 6) quartered (or cut into small pieces) cherry tomatoes.
Butter or spray 4 muffin cups in 12-cup muffin pan with non-stick vegetable spray. Divide spaghetti evenly among prepared cups. (For fun, you can also add a few cubes of mozzarella cheese to each cup). Ladle egg mixture over, filling almost to top.
Bake at 375ºF until puffed and golden, about 15-18 minutes.