Preheat oven to 375°F.
Squeeze grated zucchini inside cheesecloth to remove excess water. Set aside.
Add portobello mushroom, olives, and spinach to food processor with the steel blade attachment. Pulse until minced and combined, but be careful not to over-process, we don't want mush!
Switch food processor blade out for the plastic dough blade. To the mushroom-olive-spinach mixture add the remaining patty ingredients (including zucchini). Mix until combined and a dough can be formed, but again, be careful not to over-process. A little texture is good!
Form your dough into 4 burger patties and place onto a parchment-lined baking sheet (or a lightly oiled one). Bake in oven for 10 minutes. Flip patties and cook another 10 minutes. Remove from oven.
Change oven setting to broil. Top burger patties with vegan mozzarella. Add hamburger buns to baking sheet, cut-side-up. Place baking sheet with patties and buns under your broiler and watch carefully to prevent burning. Once buns are toasty and cheese is melted, remove from oven.
In a lightly greased skillet (or on a grill), over medium heat, add roasted red bell peppers and heat 1 - 2 minutes on each side, until hot.
Spread a light amount of vegan mayonnaise on the inside of each bun half. On top each bottom bun add a burger patty, 4 basil leaves, a roasted red bell pepper, and finally the top bun. Serve with fries, salad, or your favorite burger side!