Miso mushroom & halloumi burgers

Name: Miso mushroom & halloumi burgers
Prep time: 20 min
Cook time: 30 min
Total time: 50 min
Cuisine: American
Ingredients: 4 large portobello mushrooms stalks removed; 1 large red onion cut into 4 thick slices; 250g block halloumi cut into 8 thick slices; 4 burger buns split; 2 tomatoes sliced; 4 handfuls of rocket or other peppery leaves; burger sauce of your choice, to serve; 100g softened butter; 2 tbsp white miso; 2 tbsp light soy sauce; 1 large garlic clove grated