Preheat oven at 200 C (@ the top burner setting), 10 mins before using it.
Heat oil in a pan and add chopped onion. When it becomes soft, no need to brown it, add sliced mushrooms, salt and pepper powder. Cook till the mushrooms are done.
Slice the french roll into 1/2 inch thick slices. Apply butter on one side (you can apply on both sides, if you wish). Top the buttered side with cooked mushrooms. Sprinkle the grated cheese on top of the mushrooms.
Line a baking sheet with parchment paper and place the prepared slices on it. Reduce the oven temp to 190 C. Keep the baking tray in one of the upper racks of the oven and bake for 3-5 mins, till the cheese is melted.
Notes
Be careful while adding salt to mushroom,since you are already using salted garlic butter and there is salt from cheese too.You can use Mozzarella or Cheddar instead of Parmesan.You can make it without mushroom and it will be like Garlic bread with cheese. Also, you can use different fillings like chicken in white sauce or a combination of spinach and mushroom in white sauce etc;To make it on stove top: Make sure you butter both sides of the bread. Heat oil in a pan and place the prepared slices with cheese. Cover with a lid and cook for a min or two till cheese melts. Make sure you keep a close watch, otherwise the bottom of the bread may get burnt.