Drain and rinse canned black beans well. In a large bowl, mash 1 1/2 cups cooked black beans with a fork or potato masher leaving some texture (about 80% mashed). Pulse 3/4 cup rolled oats in a blender or food processor until coarse flour forms.

Sauté aromatics

Heat 1 tablespoon olive oil in a skillet over medium heat. Add 1/2 cup finely chopped onion and 1/2 cup finely chopped bell pepper and cook 4–5 minutes until softened. Add 1 teaspoon minced garlic (or the 1 tsp listed) and cook 30 seconds until fragrant. Remove from heat and let cool 2–3 minutes.

Mix patty base

To the mashed beans add the ground oats, sautéed vegetables, 1/2 cup grated carrot, 2 tablespoons chopped cilantro or parsley, 1 tsp ground cumin, 1 tsp smoked paprika, 1 tsp salt, 1/2 tsp black pepper and 1 tablespoon soy sauce. If using flax egg or regular egg, add it now. Mix thoroughly with a spoon or your hands until evenly combined. The mixture should hold together when pressed; if too wet add up to 2 tablespoons extra oats, if too dry add 1 tablespoon water or olive oil.

Form patties

Divide the mixture into 4 equal portions (about 3/4 cup each). With lightly oiled hands, form each portion into a compact patty about 3/4-inch thick. Press the edges to seal and smooth the surface so they don't crumble while cooking.

Cook patties

Heat remaining 1 tablespoon olive oil in a large nonstick skillet over medium heat. Add patties (work in batches if needed) and cook 4–5 minutes per side until a deep golden crust forms. Avoid flipping too early. If using cheese, place a slice on each patty in the last minute of cooking and cover the pan briefly to melt.

Finish

Transfer cooked patties to a plate lined with paper towel to rest 1–2 minutes. Taste and adjust salt if needed.

Assembly & Toppings

Toast buns

Split and toast 4 buns cut-side down in a dry skillet or toaster until lightly golden, about 1–2 minutes.

Prepare sauce

In a small bowl combine 1/4 cup mayonnaise, 1 tablespoon mustard and 2 tablespoons ketchup. Stir until smooth. Taste and adjust seasoning or add a pinch of salt or a squeeze of lemon if desired.

Assemble burgers

Spread sauce on both cut sides of each toasted bun. Place a lettuce leaf on the bottom bun, add a veggie patty, top with 2 tomato slices, 2 red onion slices and a few pickle slices. Add cheese if using, then the top bun.

Serve

Serve immediately while patties are warm. Recommend serving with a side salad, oven fries, or sweet potato fries. Leftover patties refrigerate up to 3 days and reheat in a skillet or oven.