Heat olive oil or butter in a pan over medium heat. Add minced garlic and chopped onion, sauté until the onion becomes soft and see-through.
Add sliced mushrooms and capsicum to the pan. Cook until they are soft and any liquid from the mushrooms has evaporated.
Season with salt, pepper, and oregano or mixed herbs. Mix well and cook for another 2-3 minutes. Remove from heat.
Lay out the slices of bread. If using cheese, sprinkle a tablespoon of grated cheese on two of the slices. Spread the mushroom and capsicum mixture evenly over the cheese (if used) or directly on the bread.
Place the other slices of bread on top to form sandwiches. Heat a skillet or sandwich press over medium heat. Lightly butter the outside of the sandwiches if desired, then place them in the skillet or sandwich press. Toast until the bread is golden brown and crispy, and the cheese is melted (if using), about 3-4 minutes per side in a skillet, or until golden in a sandwich press. Cut the sandwiches in half and serve warm.
