In a small bowl, mix ground flaxseed with 2 tbsp cold water until well combined and thick. Set aside for 10 minutes.
Meanwhile, in a large bowl, mash black beans with potato masher until broken down.
Add flaxseed, onion, garlic, beet, sweetcorn, coriander, thyme, paprika, flour and soy sauce to bowl, and mix well to combine. (If mixture feels too wet, add a little more flour.) Season with salt and pepper if needed.
Divide black bean mixture into 4 balls, and shape into burger patties about ¾-inch thick.
Heat 2 tbsp oil in a large frying pan on medium heat. Fry burgers for 4-5 minutes each side until browned. (If pan isn't big enough, cook burgers in two batches.)
Serve immediately on burger buns with vegan mayo, lettuce leaves, tomato slices and any other favorite toppings. Enjoy!
