Heat olive oil in a skillet over medium-high heat. Add mushrooms and red bell peppers; saute until mushrooms are soft and meaty, about 20 minutes. Add pinto beans, taco seasoning mix, green onions, and water. Stir well; continue to cook until all liquid is evaporated, about 5 minutes more.
Divide mushroom filling evenly between tortillas. Add your favorite taco toppings.