Place all the ingredients in a blender and blend until smooth.
Place the steak and marinade in a plastic bag. Seal the bag and refrigerate for at least 3 hours, up to overnight.
Heat a large skillet on medium-high heat and warm 1 tablespoon of the olive oil.
Add the bell peppers, onion, salt and pepper, and cook until tender, about 8 to 10 minutes.
Transfer to a plate.Using a damp paper towel, wipe out the skillet. Keeping the heat on medium-high, warm 1 Tablespoon olive oil.
Remove the steak from the marinade and cut into 1/2-inch-thick slices.
Place the steak in the skillet. Cook, turning once, 3 to 4 minutes per side for medium-rare, or until done to your liking.eturn the peppers and onion to the skillet and stir together to warm through.To assemble the fajitas, place a warmed tortilla on a plate.
Add a few slices of steak, some peppers, onion and sour cream, if using, and sprinkle with cilantro.
Serve with rice, beans, guacamole and salsa on the side