Step 1: Prepare the Peppers - Preheat oven to 375°F (190°C). Slice tops and bottoms off bell peppers and remove seeds. Arrange upright in a baking dish.
Step 2: Cook the Rice - If using brown rice, cook according to package instructions. For frozen cauliflower rice, thaw in the microwave.
Step 3: Sauté the Vegetables - Heat olive oil in a large skillet over medium heat. Sauté chopped onion until translucent, about 3-5 minutes. Add mushrooms and cook until tender, about 7-8 minutes.
Step 4: Combine the Filling - In a mixing bowl, combine sautéed onion and mushrooms with cooked brown rice (or cauliflower rice), Greek seasoning, black pepper, and crumbled feta. Mix well.
Step 5: Stuff the Peppers - Fill each pepper with the mixture, pressing down gently to pack tightly.
Step 6: Bake - Cover the baking dish with foil and bake for 30 minutes. Remove foil and bake for an additional 10-15 minutes, until peppers are tender and tops are golden.