Deluxe double-decker beef burger

Ingredients

• 1 kilogram beef mince
• 1 cup packaged wholemeal breadcrumbs
• 3 teaspoons dried basil leaves
• 9 eggs, at room temperature
• 2 tablespoons vegetable oil
• 2 medium brown onions, cut into rings
• 6 damper-style bread rolls
• 60 grams butter, at room temperature
• 1/3 cup mayonnaise
• 1/4 cup sweet chilli sauce
• 50 grams baby rocket leaves
• 6 slices cheddar cheese
• 3 medium tomatoes, sliced
• 225 grams can sliced beetroot, drained, patted dry with paper towels
• 1 lebanese cucumber, cut into ribbons