Preheat oven to 425ºF. Spray a large rimmed baking sheet with cooking spray or 1 tbsp olive oil. Set aside.
In a medium bowl, combine ground beef, ground sausage, egg, breadcrumbs, parsley, basil, salt, pepper, and crushed red pepper flakes. Mix until well incorporated.
Use a small cookie scoop or tablespoon to make about 40 meatballs. Lightly roll into balls and place on the prepared baking sheet.
Bake meatballs for 15 minutes. Remove and set aside.
Soup
In a large stock pot over medium-high heat, add the olive oil, carrots, onions, and celery. Saute until softened and onions are translucent. Stir in garlic until fragrant, about 1 minute.
Add the chicken stock, pasta, and season with salt and black pepper. Bring to a slight boil and cook until pasta is al dente. Add the meatballs and any juices that accumulated on the baking sheet. Lower temperature to low.
Simmer until meatballs are warmed through. Add spinach and lemon zest, stirring to combine.
Serve with freshly grated parmesan cheese and more lemon zest, if desired.
Notes
Tips for storing
Store in an airtight container in the fridge for up to 4 days.
Pasta swells in broth. For best texture, cook pasta separately and add to each bowl.
Reheat on the stove over medium heat. Add a splash of stock or water to loosen.
Freeze up to 3 months. Freeze without pasta for the best result.
Thaw overnight in the fridge, then reheat gently and add fresh spinach if you want a brighter finish.