Heat a large non stick pan over medium-low heat and add 1 tbsp butter. When the pan is heated, add the sliced onions and a pinch of salt.
Cook the onions for about 20 minutes or until they start to brown then remove them from the pan.
Increase the heat to medium and add the cooking oil. Add the sliced portobello mushrooms to the pan and cook for about 10 minutes, or until they have released their moisture and begun to brown.
Add the tamari, and a splash of water if the pan is getting dry, then add the peppers and cooked onions to the pan. Saute for about 5 minutes, or until the peppers have become tender. Season well with sea salt and pepper.
Heat your oven to 400℉
Mix the crushed garlic into 1.5 tbsp butter and brush onto both sides of the sub roll. Bake in the oven for 5 mins or until the butter melts and the roll begins to toast.
Remove the roll from the oven and top with the mushroom mixture then cover it with slices of the provolone cheese.
Place the roll back in the oven and toast until the cheese melts.
Remove from the oven and serve!
Line each roll with provolone slices and top with the mushroom mixture.