Adjust rack to middle and top positions and preheat oven to 400 degrees. Bring a large pot of salted water to a boil. Wash and dry all produce. Halve, peel, and dice onion; mince a few pieces until you have 2 TBSP (3 TBSP for 4 servings). Halve mozzarella; dice one half into four equal-sized pieces (eight pieces for 4); tear remaining half into bite-size pieces.

In a large bowl, soak half the panko (you’ll use the rest later) with 2 TBSP water (3 TBSP for 4 servings). Let sit until absorbed, then add beef, minced onion, half the Tuscan Heat Spice (you’ll use the rest later), 3/4 tsp salt (11/2 tsp for 4), and pepper. Mix to combine.

Lightly oil a baking sheet. Form beef mixture into four 3-inch-wide rounds (eight rounds for 4); divide diced mozzarella between the centers of each. Gently fold meat around cheese, shaping and sealing to create cheese-stuffed meatballs. Place on prepared sheet. Bake on middle rack until cooked through, 20-22 minutes. Remove from oven; heat broiler to high or oven to 500 degrees.

Once water is boiling, add cavatappi to pot. Cook until al dente, 9-11 minutes. Drain and set aside. Reserve empty pot. Meanwhile, place 1 TBSP butter in a small microwave-safe bowl. Microwave until melted, about 30 seconds. Stir in 1/4 tsp Tuscan Heat Spice (1/2 tsp for 4 servings; you’ll use the rest later), remaining panko, a large drizzle of olive oil, salt, and pepper.

Meanwhile, heat a large drizzle of olive oil in a large, preferably ovenproof, pan over medium-high heat. Add diced onion; cook, stirring, until softened, 4-6 minutes. Stir in remaining Tuscan Heat Spice, crushed tomatoes, tomato paste, 1/2 cup water (3/4 cup for 4 servings), 1 tsp sugar (2 tsp for 4), and salt until combined. Bring to a simmer; cook until slightly thickened, 3-4 minutes. Stir in 1 TBSP butter (2 TBSP for 4) until melted. Season generously with salt and pepper.

Carefully pour sauce into pot used for pasta; add cavatappi. If pasta seems dry, add a splash of water. Gently stir in meatballs to coat. Carefully transfer everything back into pan used for sauce. (TIP: If your pan isn’t ovenproof, transfer mixture to a 9-by-13-inch baking dish.) Top mixture with panko and torn mozzarella. Broil or bake on top rack until panko is golden brown and cheese is melted, 2-4 minutes. Let cool slightly, then serve.