Veggie Cheeseburgers

Ingredients

• 1/3 cup (75 ml) mayonnaise
• 1/3 cup (75 ml) plain Greek yogurt
• 1/2 lemon, the grated zest only
• 1 tbsp chopped fresh chives
• 1 small red onion, sliced
• 1 tsp sugar
• 1 tsp salt
• 1 eggplant, cut into slices 1/2 inch (1 cm) thick (about 8 slices)
• 1/4 cup (60 ml) olive oil
• 1 zucchini, cut into slices 1/4 inch (5 mm) thick
• 3/4 lb (340 g) halloumi-style grilling cheese, cut into four slices 1/2 inch (1 cm) thick
• 4 hamburger buns, lightly toasted
• 4 Boston lettuce leaves
• 1 tomato, sliced