Preheat the oven to 180°C.
Sauté diced onion and salt in olive oil for 5 minutes until soft.
Add garlic for two minutes, then stir in tomatoes and vinegar.
Simmer the relish for five minutes until thickened and then set aside to cool.
Melt butter in a pan and cook sliced onions with salt for 25-30 minutes until caramelized.
Place mushrooms stalk-side up on a tray, drizzle with oil, and season.
Bake mushrooms for 10 minutes until they begin to soften.
Place a slice of mozzarella on each mushroom and bake for 5-7 minutes until melted.
Lightly toast the burger buns.
Assemble by spreading relish and layering the mushroom and onions between the bun halves.