Adjust rack to top position and preheat oven to 450 degrees. Bring a large pot of salted water to a boil. Wash and dry all produce. Halve and peel onion; finely chop one half (save the other half for another use). Mince or grate garlic. Trim and quarter zucchini lengthwise; thinly slice crosswise.

In a medium bowl, combine panko, 1/2 tsp Tuscan Heat Spice (you’ll use the rest later), and 1/4 cup water. (For 4 servings, use 1 tsp Tuscan Heat Spice and 1/2 cup water.) Let sit until water has mostly absorbed, 2-3 minutes, then add beef, half the Parmesan, 1/2 tsp garlic (3/4 tsp for 4), pepper, and salt (we used 1/2 tsp; 1 tsp for 4). Gently combine, then form into 10-12 meatballs (20-24 for 4).

Lightly coat a baking sheet with a drizzle of olive oil. Place meatballs on prepared sheet. Bake on top rack until browned and cooked through, 12-15 minutes.

Meanwhile, heat a large drizzle of olive oil in a large pan over mediumhigh heat. Add onion and a big pinch of salt and pepper. Cook, stirring, until softened, 4-6 minutes. Add zucchini and remaining garlic. Season with salt and pepper. Cook, stirring, until zucchini is tender and lightly browned, 3-4 minutes.

Once water is boiling, add spaghetti to pot. Cook until al dente, 9-11 minutes, then drain and set aside. To pan with veggies, add crushed tomatoes, stock concentrate, 1/4 cup water, remaining Tuscan Heat Spice, 1/2 tsp sugar (1 tsp for 4 servings), salt, and pepper. Bring to a boil, then reduce heat to medium. Cook until slightly thickened, 8-10 minutes. Stir in meatballs. Cook until sauce is very thick, 2-3 minutes more.

Return spaghetti to empty pot. Carefully pour in sauce, including meatballs, and 2 TBSP butter; toss to coat. If sauce seems dry, add a splash or two of water. Season with salt and pepper. Divide between plates and sprinkle with remaining Parmesan.