Preheat oven to 350°. In a large pot of salted boiling water, cook penne according to package directions until al dente. Drain.

Meanwhile, in a large oven-safe skillet over medium heat, melt 1 tablespoon of butter. Add garlic, shrimp, and parsley and season with salt. Cook until shrimp is pink and no longer opaque, 2 minutes per side. Transfer shrimp to a plate. (Keep juices in skillet.)

Add remaining 2 tablespoons butter to skillet and let melt, then stir in flour and cook 1 minute. Add milk and broth and bring to a simmer. Stir in 3/4 cup of mozzarella and 1/4 cup of Parmesan and season with salt and pepper.

Add shrimp, tomatoes, and cooked penne and toss until combined. (Add another splash of milk if mixture is too thick.)

Sprinkle with remaining 1/4 cup mozzarella and 2 tablespoons Parmesan and bake until melty, 5 to 7 minutes.

Garnish with parsley before serving.