Cheesy beef and mushroom toasted sandwich recipe

This quick and easy melt is totally moreish.
Combining the umami flavours of meat patties, cheese, onions and mushrooms with tangy rye bread delivers a luscious and filling sandwich. If you aren’t in the mood for barbecuing, then this recipe works just as well cooked in a heavy fry pan.
Serves 4
Ingredients
4 slices rye bread
2 Tbsp butter
1 red onion, cut in half moons
100g mushrooms, sliced
4 premade beef patties, frozen
8 slices of mozzarella
4 Tbsp garlic mayonnaise, optional
Salt, to taste
Method