Place the onion into the bowl of a food processor fitted with an S-blade. Pulse a few times until the onion is coarsely chopped. Add the chickpeas, walnuts, panko breadcrumbs, flaxseed, garlic, Worchestershire sauce, vinegar, soy sauce, paprika and black pepper. Pulse until the ingredients are finely chopped and well-blended. Test a bit of the mixture by pressing it together to see how well it holds; pulse a few more times if it seems too crumbly. Shape into 4-6 patties.
Lightly oil a large skillet and place it over medium heat. Working in batches if needed, place the burgers into the skillet and cook them until they're browned on the bottom, about 4 minutes. Flip and cook until browned on the opposite sides, about 4 minutes more.
Stir the horseradish mustard and maple syrup together in a small bowl.
Stuff the buns with the burgers and top with the maple horseradish mustard, lettuce, tomato and onions, then serve.