Using a food processor or blender, add all of the marinade ingredients; pulse until smooth.

Place skirt steak in a large re-sealable bag. Add marinade, turning several times to coat. Refrigerate for 1 hour.

Preheat grill to medium-high heat.

Remove skirt steak from bag; discard any leftover marinade.

Brush olive oil on both sides of the sliced onions.

Place steak and onions on grill. Grill for 3-5 minutes per side, or until steak is done to taste.

Reduce grill temperature to medium heat.

Transfer skirt steak and onions to a large cutting board and let it rest for 5 minutes. Slice skirt steak against the grain into strips. Slice onions into strips.

Lay tortillas out and split shredded cheese, sliced onions and steak evenly between tortillas. Fold each tortilla in half.

Grill quesadillas for 2-3 minutes per side, or until cheese has fully melted. Cut quesadillas into wedges and serve with salsa and sour cream.