Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper or a silicone baking mat.
Tear the slices of bread into quarters and place them in your food processor. Process until you have small bread crumbs, about 20 seconds. Transfer to a large mixing bowl and set aside for now. (No need to wash the food processor bowl.)
Place the kidney beans in your food processor. Pulse about 10 times, until they’re broken down into smaller pieces. Transfer them to the mixing bowl with the bread crumbs. (No need to wash the food processor bowl.)
To the food processor, add the onion, red bell pepper, walnuts, tomato paste, basil, balsamic vinegar, Worcestershire sauce, ground flax, tapioca flour, oregano, smoked paprika, garlic powder, salt, black pepper, and red chili flakes. Pulse 10 times and then scrape down the sides of the food processor. Pulse another 10 times until everything is mixed, but not mushy. Transfer to the mixing bowl with the beans and bread crumbs. Stir until everything is mixed well. The mixture should be a little sticky but not wet and falling apart.
Form six burgers (about ⅓ packed cup / 95 g each) and place them on the baking sheet. Bake for 20 minutes then carefully flip the patties over and bake for 15 - 20 additional minutes. Allow to cool at least 10 minutes before removing from the baking sheet.
Top with your favorite pizza toppings, add a bun, and enjoy!