1 (24-ounce) jar marinara
3 tbsp. grated Parmesan, plus more for serving
3 tbsp. panko breadcrumbs
1/3 c. chopped fresh flat-leaf parsley
2 cloves garlic, pressed
Kosher salt and freshly ground black pepper
1/2 lb. sweet Italian sausage, casings removed
1/2 lb. ground beef
1 c. baby spinach, chopped
1. Pour marinara into a 5- or 6- quart slow cooker. Line a rimmed baking sheet with aluminum foil.
2. Heat oven to broil with oven rack 4 inches from heat. Toss together Parmesan, panko, parsley, and garlic in a bowl. Season with salt and pepper. Add sausage, beef, and spinach; gently mix to combine. Shape into 20 balls (about 1 1/4 inches); place on prepared baking sheet. Broil until lightly browned, 2 to 3 minutes.
3. Place meatballs in marinara in slow cooker. Cook, covered, until meatballs are cooked through, 3 to 4 hours on high or 5 to 6 hours on low.