Heat coconut oil in a pan over medium high heat. Add mushrooms and do not touch for 5-8 minutes. Flip and cook (without touching) for another 5 minutes.
Add onion (and any other veggies you want, like chopped zucchini or carrots…) and sauté until translucent, about 3-5 minutes.
Add beef (optional) and garlic. Saute the beef for 8 minutes making sure to break it apart with a spatula.
Add half a jar of all natural marinara sauce. Mix everything together and put the lid on and cook on low heat for 10 minutes.
Stir in spinach and add the salt, pepper and red pepper flakes (omit red pepper if you do not like spice). Put the lid back on and cook for 5 minutes.
Taste the mixture and add additional salt or pepper if needed.
Place hollow (tops removed) peppers in an oven safe dish that will allow them to stand up.
Spoon the meat mixture into the bell peppers.
Place in the oven for 15-20 minutes. If you would like to top with cheese, sprinkle on top after 15 minutes and let bake for an additional 5 minutes.