Heat the oil in a large skillet over medium heat. Add the onions and cook, stirring occasionally, until tender and beginning to brown at the edges, about 10 minutes.
Add the garlic and cook until fragrant, about 2 minutes. Add the steak to the skillet and cook until browned, but still rare, about 5 minutes.
Stir-in the mushrooms and cook for 3 minutes, or until mushroom wilt and release moisture. Add the bell pepper and sprinkle the lime over everything. Allow to simmer uncovered for 2-3 minutes, stirring occasionally.
Peppers should be only tender, not soft. Sprinkle evenly with 1 tablespoon of Fajita Mix Seasoning and a couple tablespoons of water; stir well to coat with the seasoning and cook another minute. Taste and add more seasoning, as desired. If pan seems too dry add a little more water.
Warm flour tortillas in the microwave for 1-2 minutes in a tortilla warmer to soften or in a dry skillet over medium-high heat for 30 seconds each side.
Warm corn tortillas in a dry skillet over medium-high heat for 30 seconds on each side to soften. Fill the warmed tortillas and garnish with sour cream, guacamole, and salsa.
Time Saver: The beef can be prepped and frozen in advance. The remaining ingredients can be prepped up to three days in advance.Variations: