Cheese and harissa onion toastie

Ingredients

• 0 2 tbsp olive oil
• 0 1 red onion, finely sliced
• 0 1 white onion, finely sliced
• 0 1 tbsp harissa (I like rose harissa)
• 0 1 tbsp mayo
• 0 1 clove garlic, minced
• 0 2 slices of sourdough
• 0 4 slices Leerdammer
• 0 Knob of butter – 5g
• 0 Tbsp rapeseed oil