Add cumin, chili powder, garlic salt, pepper and red crushed peppers to a large sealable baggie. Seal and shake to mix.
Pour in lime juice and shake until combined.
Place skirt steak into baggie, seal, shake and place in fridge for 4+ hours, turning baggie over a couple times while marinating.
Using a large cast iron skillet, heat skillet over medium high heat and add 2 tablespoons of oil to pan.
Once oil is hot, add bell peppers and onions. Toss with tongs and cook until peppers are tender but still a bit crispy, approximately 6-8 minutes. Remove to a plate and set aside.
Add remaining 2 tablespoons of oil to pan and once oil is hot, remove steak from baggie and place in hot skillet. Discard liquid.
Cook the meat on medium high for 4-6 minutes per side for medium.
Remove from skillet and place on plate, cut into thin strips, against the grain, and either place back into skillet along with peppers and onions, stir to combine or serve individually with other ingredients.
Place fajitas into warm tortillas and top with (optional), guacamole, charro beans, sour cream, cilantro, Pico de Gallo.