1. Preheat the oven to 425 degrees Fahrenheit.
2. Cook the rice: Start the rice to cook.
3. Pre-bake the bell peppers: Wash the peppers and cut them in half lengthwise; remove the stems and seeds. Line a baking sheet with parchment paper, then place the peppers on top. Bake with the cut side down for 15 minutes. Remove from the oven and flip to cut side up, draining any with water. Sprinkle with salt (about a heaping ¼ teaspoon kosher salt divided between the peppers). Bake another 15 minutes until tender, then remove and stuff (see below).
4. Meanwhile, make the filling: Roughly chop the mushrooms. Place them in a large food processor, then add the walnuts and tamari, ketchup, spices, and salt. Pulse gently multiple times until everything is evenly chopped, being careful not to over process (or it will be too mushy). If you don’t have a food processor, use a chef’s knife to roughly chop everything and place in a bowl with the spices.
5. Heat 1 tablespoon of the olive oil in a non-stick skillet over medium low heat. Add the walnut filling and cook for 7 to 10 minutes, stirring until it releases all of its liquid then becomes dry and browned. Stir in the remaining 1 tablespoon olive oil. Ground beef variation: Drizzle olive oil into a skillet. Add the beef with the spices and salt and pepper. Cook for about 8 minutes until browned, then drain the fat. Taste and add additional salt as needed.
6. Stuff the peppers: Once the peppers are pre-baked, mix the filling from the previous step with 2 cups cooked rice (measure it), pizza sauce, fresh mozzarella cheese, shredded mozzarella cheese, and kosher salt. (If desired, add a mix-in like drained capers or chopped olives; see above.) Taste and add additional salt if necessary. Spoon the filling into the peppers.
7. Bake the peppers: Top with a handful of shredded mozzarella, and grated Parmesan and/or panko or breadcrumbs. Place the peppers back onto the parchment lined baking sheet and bake 20 minutes until the cheese melts. Allow to cool a few minutes before serving.