Pour the pasta into a large skillet (about 12 inches in diameter). Pour 3 cups cold water into the pan; the water should cover the pasta.

Turn the stove burner to high and set a timer for 10 minutes. Cook on high, stirring frequently, until most of the water is absorbed, about 10 minutes. Reduce heat to medium, add milk, and simmer for an additional 3 minutes, stirring constantly so the milk doesn't burn. Remove from the heat, add the salt, pesto, and 2 cups grated cheddar cheese, stirring until the cheese is melted. Preheat the broiler.

Sprinkle the top of the pasta with the remaining ½ cup cheddar cheese and place the sliced tomatoes on top of the cheese. Sprinkle the tomatoes with parmesan cheese. Place under the broiler for a minute or two, until the parmesan cheese melts and turns golden brown. Serve immediately.