Place portabella slices on a baking sheet and bake in the oven until tender, about 10 minutes.
4 portabella mushrooms
While the portabellas are baking, heat the olive oil over medium heat in a large skillet.
1 tablespoon olive oil
Add the onion and saute for about 2-3 minutes.
1 cup chopped onion
Add the button mushrooms and stir. Cover and cook about 10 minutes, stirring occasionally, until mushrooms are tender.
16 ounces sliced button mushrooms
When the portabellas are tender, remove them from the oven and add them to the skillet.
Add the salt and stir to combine. Reduce heat to low and cover to keep warm.
¼ teaspoon salt
Place two slices of provolone on each hoagie roll.
8 slices provolone or American cheese, 4 hoagie rolls
Optional: Place the hoagie rolls with cheese on a baking sheet and place in the oven (which should still be hot from the portabellas) for 3-4 minutes to melt the cheese and lightly toast the rolls.
Distribute the mushroom/onion mixture over the cheese and serve.
Notes
Nutrition information is approximate. Your results may be different due to different brands/products and measuring differences.