Pre-heat the oven to 400°F.
Wash then trim the tops off of each pepper and discard any seeds and trim any white membranes. Place the peppers onto 13 x 9 baking pan.
In a large skillet brown the ground beef until no longer pink. Drain the fat and remove the ground beef to a bowl and set it aside.
Add the olive oil to the same skillet you used for frying the ground beef, once warmed add the onion and sauté for 1 minute, then add the garlic and sauté for 30 seconds.
Add the ground beef back into the pan along with the diced tomatoes, salt, black pepper, garlic powder, and dried thyme. Mix until combined.
Add in the riced cauliflower breaking it up if it's in frozen pieces. Simmer for ten minutes, if it seems dry add up to 1/2 cup of water. You want the mixture moist but not super runny.
Add in the spinach and stir everything to combine, letting the spinach wilt.
Stuff each pepper until almost full. Then top them with the mozzarella cheese.
Add 1/2 cup of water to the bottom of the baking pan.
Cover with aluminum foil and bake for 30 minutes or until peppers are tender. You want the peppers to be somewhat soft but still hold their shape.
Remove the aluminum foil and bake a few more minutes to get the mozzarella cheese a bit brown if desired.
Remove the peppers from the baking pan discarding any liquid at the bottom.
Place the peppers onto individual bowls or plates to serve.