Beetroot and lentil burgers

Ingredients

• 2 medium beetroot, peeled, grated
• 400 grams can lentils, rinsed, drained
• 200 grams feta cheese, crumbled
• 3 green onions, thinly sliced
• 1 cup fresh bread crumbs
• 2 eggs, lightly beaten
• 1 tablespoon chopped mint leaves
• 1/2 teaspoon sumac
• 2 tablespoons olive oil
• 4 bread rolls, split
• 2 tomatoes, sliced
• 1 red onion, thinly sliced
• 1/2 cup cup mixed lettuce leaves
• 1/4 cup harissa mayonnaise