Prep ingredients: Preheat the oven to 425°F with a rack in the center. Peel and finely chop 2 large cloves garlic. Halve fontina horizontally, then cut into 8 pieces total. Chop tomatoes in their can with kitchen shears. Cut roll into 1-inch pieces. On a sheet of foil, toss bread cubes with 1 tablespoon oil and ¼ teaspoon each salt and pepper.
Sauté spinach: Heat 1 teaspoon oil in a medium ovenproof skillet, preferably nonstick, over medium-high. Add ½ of the spinach, ⅓ of the garlic, and 1 tablespoon water, and cook until wilted, 1–2 minutes. Transfer to a fine-mesh sieve and press out excess liquid. Coarsely chop wilted spinach on a cutting board, then transfer to a medium bowl.
Make meatballs: To bowl with spinach, add turkey, panko, 1 large egg, 1 tablespoon oil, ¾ teaspoon salt, and ¼ teaspoon pepper. Using lightly moistened hands, form into 8 meatballs. Press a piece of cheese into the center of each meatball and patch the hole, reforming into a ball.
Brown meatballs: Heat 2 tablespoons oil in the same skillet over medium-high. Add meatballs and cook, turning occasionally, until browned but not cooked through, about 6 minutes. Transfer to a plate leaving oil in the pan.
Braise meatballs: Add remaining garlic and 1 teaspoon chili powder to same skillet and cook 10 seconds. Add tomatoes with their juice, ⅓ cup water, ½ teaspoon salt, several grinds pepper, and ½ teaspoon sugar. Boil for 2 minutes. Add meatballs, turning to coat and place skillet in oven. Cook, turning meatballs once, until cooked through and sauce is thickened, 15-20 minutes.
Make salad & serve:While meatballs are cooking, transfer croutons to oven and toast directly on rack next to meatballs until toasted, 10–15 minutes (watch closely as ovens vary). In a medium bowl, toss remaining spinach, vinegar, 1 tablespoon oil, and croutons. Season with salt and pepper and serve alongside meatballs and sauce
Notes
Allergens: Wheat, Milk and Soy. May contain traces of other allergens. Packaged in a facility that packages gluten containing products.