Preheat the oven to 325 degrees.
Wash all the bell peppers. Using a sharp knife, cut the tops off and remove any seeds and membranes and discard.
In a large sauté pan over medium-high heat, crumble the ground beef. Add the squash, onion, garlic, and cauliflower. Cook, stirring often, until the beef is no longer pink and the vegetables are softened, approximately 8 to 10 minutes.
Drain the meat mixture in a colander and allow the excess grease to drain out. You can gently press the mixture against the sides of the colander with a spoon or spatula if you’d like to remove more fat.
Add the meat and vegetables back into the sauté pan. Add the crushed tomatoes, spinach, salt, onion and garlic powders, ground black and white pepper (if using), and paprika. Gently stir and allow to come to a gentle simmer over medium heat. Once the spinach has wilted, taste for seasonings and adjust as desired.
Fill each bell pepper with the meat mixture to the top, mounding if necessary. You might have enough to mound the filling in each pepper. Place the stuffed peppers into your braiser, cut side up.
Pour the beef stock into the bottom of the braiser and place the lid on top. (Depending on the size of your cookware, you may not need the whole 3 cups. You want your braising liquid to come at least 2 inches up the sides of the peppers.)
Being careful not to spill the liquid, place the braiser into the preheated oven for 1 hour. Using a fork or a sharp knife, check the doneness of a pepper. If it is softened, remove them from the oven. If necessary, cook another 10 minutes. You may have to adjust heating times due to each oven’s unique heating element.
When the bell peppers are softened, remove from the oven and turn on the broiler. Sprinkle with the shredded cheese and place under the broiler to melt the cheese and get a little browned if you’d like.
Remove the stuffed peppers from the oven and sprinkle with the green onions.
You can serve immediately from the braiser, or gently remove the peppers and place them on individual plates or a large serving platter. Serve warm from the oven.
See “Notes” for ideas for the braising liquid.