Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper. Prepare the peppers and place them cut side up on the baking sheet. Set aside. If you do not have cooked brown rice on hand, prepare it according to package instructions.
In a large skillet, heat the olive oil and garlic over medium heat. Add the onion and zucchini, and sauté until soft, about 5-7 minutes. Add the spinach, diced tomatoes, basil, oregano, and cooked brown rice to the skillet. Cook for another few minutes, or until the spinach is completely wilted.
Remove the skillet from heat, and put a large spoonful of the filling into each pepper half. Top the halves with a sprinkle of mozzarella cheese.
Put the peppers in the oven and bake for 20-25 minutes. Enjoy immediately, or keep in the fridge for up to 5 days in an airtight container. Reheat in the microwave for 1-2 minutes or in the oven at 300 degrees F for 10 minutes.
Substitutions: you can sub any whole grain for the brown rice (quinoa or farro would work well); to make these vegan, omit the cheese