Set your oven to 350°F (175°C). Prepare a baking sheet by lightly greasing it or lining it with a silicone baking mat.
In a skillet, heat a little oil over medium heat. Add the chopped onions and cook for about 7-8 minutes until they become soft and golden brown. Once done, transfer the onions to a bowl and let them cool.
In a food processor, pulse the walnuts until they are roughly chopped – you want them to retain some texture, so avoid over-processing. Add the cooked black beans, cooled onions, garlic powder, smoked paprika, red chili powder, ground cumin, mushroom powder, salt, and pepper. Pulse until the ingredients are just combined, retaining a slightly chunky texture.
Transfer the mixture to a large bowl. Add the flour gradually and mix until it forms a rough dough. If the mixture feels too sticky, add a little more flour. Lightly knead the dough with damp hands, then shape it into patties about 3/4-inch thick.
Place the patties on the prepared baking sheet. Bake in the preheated oven for about 10 minutes on each side or until the patties are firm in the center and crisp on the outside. Remove from the oven and set aside.
Stir the cornstarch slurry into the sauce and continue cooking for about 20 seconds, stirring constantly, until the sauce thickens further. Add the baked patties to the pan, gently flipping them to ensure they are evenly coated in the sauce. Sprinkle the sesame seeds on top. Remove from heat once the patties are fully coated and the sauce has thickened.
To serve, place the saucy patties on burger buns spread with vegan garlic mayo. Add lettuce, tomato slices, and onions as desired. Top with the remaining bun.
Close the burgers and enjoy your spicy and saucy vegan bean burger!