Preheat the oven to 350ºF (180ºC).
Prepare the peppers: Wash, cut off the tops (set aside) and remove the seeds and membrane. Place the peppers cut-side up in a baking dish just large enough to hold them upright. Finely chop the tops and set aside.
Heat 2 tbsp of oil over medium-high heat in a large skillet. Add the beef, season with salt and pepper and cook, breaking up the lumps, until the meat is just beginning to brown. Move to a paper towel-lined plate to drain fat.
Carefully wipe out the skillet and add the remaining oil. When it’s heated, add onions and reserved chopped pepper tops and cook until soft, 4 to 5 minutes. Add garlic and zucchini and cook for another minute. Add the tomatoes, stir, and season with salt, pepper, and a splash of sriracha.
Cook until everything is heated through, then stir in the beef and quinoa/barley/cooked rice cauliflower. Taste and adjust the seasoning. Stir in ½ cup (75 g) of the cheese.
Fill the peppers with the beef mixture and top each with a sprinkle of the remaining cheese.
Pour a small amount of water into the bottom of the baking dish and drizzle the peppers with a little olive oil.
Cover the pan with foil and bake for 30 minutes, then remove the foil and bake for another 15-20 minutes. The peppers should be soft.
Serve with a side salad. Enjoy!