Heat oil in a large skillet set over medium-high heat. Sauté beef, mushrooms, onions, pepper and fajita seasoning mix until browned. Add water and bing to a boil. Remove from heat. Add cilantro and half the green onions.
Preheat oven to 375°F place half of the tortilla chips onto the bottom of a large heat-proof glass dish. Layer with 1/3 of the cheese and 1/2 of the fajita beef. Repeat. Then top with remaining 1/3 of the cheese. Bake until cheese is bubbly, about 8 minutes. Top with remaining green onions.