Preheat oven to 350 degrees F. Line a baking sheet with foil; set aside.
In a large mixing bowl, stir together oil and fajita seasoning. Add potatoes and mushrooms and toss until well coated. Transfer to prepared baking sheet and arrange in a single layer. Sprinkle with cheese and bake for 15 minutes. Remove and set aside.
Meanwhile, stir together beans and salsa in microwave-safe bowl. Cover with a damp paper towel and microwave on HIGH for 3 minutes; set aside.
Heat taco shells in oven for 3 to 5 minutes.
Spoon beans into heated taco shells. Top with potato mixture and garnish with red onion, tomato, and lettuce.