Make the marinade: In a blender, combine all the marinade ingredients and blend until smooth. Taste and adjust seasoning as needed.
Place the thinly sliced skirt steak in a large bowl or zip-top bag and pour the marinade over the top. Toss to coat evenly, cover, and refrigerate for at least 8 hours or overnight.
Remove the steak from the marinade and discard the excess liquid. Pat the steak dry with paper towels.
Heat a large cast iron skillet over medium-high heat until very hot. Working in batches to avoid crowding, cook the steak for about 4 minutes per side until deeply caramelized and crispy on the edges. Transfer to a cutting board and let rest for 5 minutes while you cook the vegetables.
In the same skillet with all the steak drippings, add the bell peppers and onions. Sauté over medium-high heat for 8 to 12 minutes, stirring occasionally, until the vegetables are softened and starting to char at the edges.
Add the steak back into the skillet with the peppers and onions. Toss to combine, remove from heat, and serve immediately with warm tortillas, guacamole, and salsa.
