Preheat the oven to 375°F (190°C).
Prepare the bell peppers by slicing off the tops and removing the seeds and membranes. Set aside.
Heat olive oil in a large skillet over medium heat. Add the onion and sauté for 3–4 minutes, until softened.
Add garlic, zucchini, and mushrooms, cooking for another 5 minutes until tender.
Stir in the black beans, cooked rice (or quinoa), diced tomatoes, tomato paste, and all spices. Cook for another 3–5 minutes until everything is heated through.
Taste and adjust salt, pepper, and spices as needed.
Stir in fresh herbs (parsley or cilantro) and remove from heat.
Stuff each bell pepper with the mixture, packing it in tightly.
Arrange the peppers upright in a baking dish. If desired, sprinkle vegan cheese on top.
Cover with foil and bake for 30 minutes. Remove foil and bake for an additional 10 minutes to brown the tops slightly.
Let cool slightly before serving.