Preheat the oven to 350 degrees F. and line a baking sheet with parchment paper. Tear the bread into 1" pieces and add them to the baking sheet in a single layer.
Bake for 7-10 minutes, flipping halfway through. Keep an eye on the bread during the last 2 minutes so it doesn't burn.
Once the bread has cooled, add it to a food processor. Pulse until it is a breadcrumb consistency. In a large bowl, mix together the ground beef, egg, Parmesan, breadcrumbs, garlic, Italian seasoning, onion powder, salt, and pepper. Form into twenty 1" meatballs.
Heat a large nonstick pot over medium heat. Add the meatballs and brown on all sides for about 4-5 minutes (they don't need to be fully cooked). Remove and set aside.
Remove any excess grease from the pan. Add two teaspoons of olive oil to the pan, then add onion, celery, carrot, and garlic. Cook 3-4 minutes until softened and fragrant.
Add chicken broth, Italian seasoning, and crushed red pepper flakes. Stir to combine, then return the meatballs to the pot.
Stir in the riced cauliflower and simmer uncovered for 15-20 minutes, or until the meatballs are cooked through and the cauliflower is tender.
Stir in the spinach and cook just until wilted, about 1-2 minutes.
Ladle into 4 bowls and serve hot.