Lentil cakes with Harissa yogurt

Ingredients

• 2 cups reserved lentils , (from smoky braised lentils)
• 1/4 cup bread crumbs
• 1 egg
• 1/4 tsp salt
• 1 tsp olive oil
• 2/3 cup Greek yogurt
• 4 tsp harissa paste
• 4 6-in. multi-grain pitas
• 1/4 English cucumber , halved and thinly sliced
• 4 green leaf lettuce leaves
• 1 plum tomato , thinly sliced into rounds
• 1/4 red onion , thinly sliced