Mix flaxseed with water to make a flax egg. Let it sit for 5 minutes.
Combine mashed lentils/chickpeas, breadcrumbs, onion, paprika, flax egg, salt, and pepper to form burger patties. Fry in a pan until golden on both sides.
Mix all the sauce ingredients together.
Toast the buns. Spread Big Mac sauce on both sides. Add the patties, vegan cheese, lettuce, pickles, and onions. Serve and enjoy!