In a food processor, combine the cilantro, onions, rolled oats, and garlic and pulse until finely chopped. Add the black beans, flaxseed, cumin, onion powder, salt, garlic powder, coriander, lime juice, and olive oil. Pulse 6 to 8 times until about half the mixture resembles a paste, but you can still see plenty of texture. Be careful not to purée the whole mixture or the burgers will not stick together.

Heat a large nonstick skillet over medium heat. Scoop a generous 1⁄3 cup (75 mL) of the bean mixture and flatten it into a patty in the pan to ¾ inch (2 cm) thick and 3 to 4 inches (8 to 10 cm) wide. Cook, in batches if needed, until a brown crust forms on the bottom, 4 to 5 minutes. If browning too fast, reduce the heat so the burgers have time to cook through without burning on the bottom. Carefully flip and cook for another 4 to 5 minutes until brown on the other side. (You can freeze uncooked patties, stored between layers of parchment paper in a resealable plastic freezer bag, for up to 1 month. Cook from frozen, until cooked through and golden brown, 7 to 8 minutes per side.)

To serve, slice open the buns and spread some mayonnaise on the bottom half of each and top with a patty, sliced or smashed avocado, and pickled red onions. Spread some of the spicy cilantro garlic sauce on each of the bun tops, then arrange them on top of the pickled red onions.