Cover couscous with the same amount of boiling water. Cover and let swell for 10 minutes. After 5 minutes, loosen with a fork.
Peel and finely chop the onion and garlic. Heat 1 tablespoon of olive oil in a pan and fry the onions until golden. Add the garlic and fry briefly.
Mash the beetle beans with a fork, peel the carrots and grate finely. Finely chop the parsley and chives.
Mix the couscous, roasted onions, mashed beetle beans, carrots and chopped herbs with the egg. Season with a squeeze of lemon juice, salt and pepper. Let the mixture rest in a cool place for at least 30 minutes.
Form 4 loaves from the mixture with wet hands and brush with oil on both sides. Grill on a hot grill for about 10 minutes until crispy, turning after 5 minutes. Alternatively, you can grill it in a pan.
Wash and slice the tomato. Wash the lettuce and spin dry. Pat the feta cheese dry if necessary and cut into slices.
Cut the buns crosswise and warm them briefly on the grill grate. Place the loaves on the buns. Top with tomato slices, arugula and feta cheese and finish with a little mayonnaise. Place the bun lid on top.