Preheat oven to 350°F/177°C.
Cut and discard tops from bell peppers and remove seeds. Place peppers in boiling water for 3-4 minutes. Remove and immediately rinse in cold water. Place the blanched peppers to the side.
In a skillet, cook ground beef over medium heat until no longer pink, breaking into crumbles - about 6-8 minutes. Remove with a slotted spoon and pour off drippings.
In the same pan, heat oil over medium heat. Saute zucchini, yellow squash, yellow onion, and chopped pepper until tender - about 4-5 minutes.
Add spinach and garlic. Cook and stir until spinach is wilted - about 1 minute.
Stir in cooked ground beef, cauliflower rice, tomato sauce, mozzarella cheese, and salt.
Place bell peppers in a baking dish. Fill each pepper with half the ground beef mixture.
Bake, covered, until peppers are tender - about 35-40 minutes.