Before starting, preheat the broiler to high.Wash and dry all produce. Add 10 cups water and 2 tsp salt to a large pot (use same for 4 ppl). Cover and bring to a boil over high heat.Add penne to the boiling water. Cook uncovered, stirring occasionally, until tender, 10-12 min.Drain and return penne to the same pot, off heat.

While water comes to a boil, peel, then cut shallot into 1/4-inch pieces. Peel, then mince or grate garlic.Roughly chop spinach. Drain, then rinse olives. Pat dry with paper towels, then roughly chop.Pat bocconcini dry with paper towels. Tear into quarters, then season with 1/4 tsp (1/2 tsp) garlic salt and pepper.

Once penne is added to the boiling water, heat a large oven-proof pan over medium heat. When hot, add 1 tbsp (2 tbsp) oil, then shallots and garlic. Cook, stirring constantly, until softened and fragrant, 2-3 min. Season with salt.

Add crushed tomatoes, olives, 1/2 tsp (1 tsp) chili flakes and 1/4 cup (1/2 cup) water to the pan with shallots. Cook, stirring occasionally, until sauce thickens slightly, 4-5 min. Remove from heat. Stir in 1 tbsp (2 tbsp) balsamic glaze. Season with 1/4 tsp (1/2 tsp) garlic salt and pepper.

Add sauce and spinach to the pot with penne, then stir to combine.Transfer penne to the same oven-proof pan. (NOTE: If you don't have an oven-proof pan, transfer penne to an 8x8-inch [9x13-inch] baking dish.)Arrange bocconcini over penne. Sprinkle Parmesan over top. Broil in the middle of the oven until cheese melts, 4-5 min.

Divide penne al forno between plates. Sprinkle remaining chili flakes over top, if desired. Drizzle with remaining balsamic glaze, if desired.