In a large frying pan, sauté onion in oil until golden.

Add peppers and sauté until softened. Stir in mushrooms and tomatoes, and sauté until all vegetables are soft. Remove from heat.

Season vegetables with salt, pepper, and garlic powder. Add 1 cup shredded cheese to the mixture, stirring to combine.

Cut a deep slit in each baguette, forming an opening until almost the bottom and both ends. Press bread down and into sides, creating room for a pocket of vegetable filling. (Alternatively, you can scoop out some of the insides to hollow out your pocket.)

Divide mixture evenly among baguette pockets.

Place baguettes onto a lined baking sheet. Sprinkle tops of baguettes with remaining shredded cheese.

Broil in oven on bottom rack for about 5–8 minutes, making sure that the cheese and rolls do not burn.

Remove from oven and serve immediately.