Preheat your oven to 400°F/200°C 

Prepare your quinoa.  Add quinoa to a fine mesh strainer and rinse under cold running water for 30 seconds. Drain well. In a saucepan, add 1 cup quinoa with 2 cups of water. Bring to a boil over medium-high heat.

Once boiling, stir the quinoa and turn the heat to low. Cover and simmer until all the liquid is absorbed, about 15 minutes. Use a fork to fluff and separate the grains.

Meanwhile, add cauliflower florets, raw walnuts, dried oregano, garlic powder, salt and pepper to a food processor. Pulse until evenly ground (see photo). Pour onto a lightly greased baking sheet. Bake for 20 minutes, stirring halfway through to prevent over-browning.

Heat olive oil in a skillet over medium heat. Add chopped onion and diced zucchini, if using. Sauté until soft, about 3-5 minutes. 

In a large bowl, add cooked quinoa, cauliflower-walnut mixture, sautéed onions and zucchini, and 2 cups marinara sauce. Mix until well combined.

Lower oven heat to 375°F/190°C. Cut the tops off the bell peppers; discard seeds and membranes. 

Spread 1 cup of marinara sauce evenly on the bottom of the pan. Stand bell peppers up in the pan. With a spoon, evenly add the stuffing to each pepper.

Cover and bake at 375°F/190°C for 45 minutes. In the remaining 15 minutes, top with vegan cheese and return pan to oven uncovered.