Pre-heat oven to 375 degrees. Clean peppers and set aside.
Dice onion and saute in olive oil. Add spinach (squeeze dry if using frozen) and let cook down. Add ground beef and brown. Dice tomato and add to mixture. Season to taste with your favorite seasonings.
Fill peppers with mixture and bake for 20 minutes or until peppers are tender.
Dig in or top with salsa or marinara sauce and enjoy.
Notes
These would be fantastic with Parmesan cheese on top for some crunchy salty bite. I'm learning to live without cheese.