Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add mushrooms and cook 3–4 minutes, or until softened and browned.
Add wine and increase heat to high. Cook until liquid has evaporated, about 2–3 minutes. Transfer to bowl.
Preheat oven to 250°F.
Carefully wipe skillet clean. Heat remaining tablespoon of olive oil over medium-high heat. Add green pepper and onions; cook until softened, about 5–8 minutes.
Reduce heat to low. Return mushrooms to skillet and stir to combine, then season to taste with salt and pepper. Top vegetable mixture with cheese slices. Once cheese has melted, remove from heat.
Divide veggie mixture into sub rolls. Wrap each cheesesteak tightly with foil and place on a baking sheet. Bake for 15 minutes. Remove from oven and unwrap carefully.
Notes
If you prefer, you can remove the gills from the mushrooms before cooking them. They have a tendency to discolor the bread and onions a little.
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