Place the skirt steak in a sous vide bag and add all the seasonings.
Remove as much air as possible from the bag, being careful not to draw out any liquid when using a vacuum sealer.
Set your immersion circulator to 130–135°F for medium-rare.
Place the steak into the water bath. Set a timer for 3 hours.
Remove the steak with a pair of tongs after the 3 hours is up.
Heat up your cast iron pan and cook the onions and bell peppers over medium heat until soft, about 10 to 15 minutes. Set aside.
Add some oil to a cast iron pan and set it over high heat. Wait until the pan is sizzling hot, then cook the steak for 30–60 seconds per side, just until browned. Remove the pan from the heat and add the peppers and onions back in.
Slice the steak against the grain at an angle and add it to the pan with the peppers and onions. Serve with tortillas and season with additional salt and pepper, if needed.