In a small bowl, whisk together all of the marinade ingredients until well combined.

Place the flank steak in a shallow baking dish. Pour marinade over steaks, turning several times until well coated. Cover and refrigerate for 4-6 hours.

Brush bottom of a large fry or sauté pan with olive oil. Add sliced onions and balsamic vinegar. Stir until onions are well coated and place pan over medium-low heat. Let cook for 25-30 minutes on medium-low heat, stirring occasionally. (Note: If onions begin to sizzle, reduce heat to low.)

Meanwhile, remove steak from the marinade. Discard marinade.

Place a large frying pan or grill pan over medium-high heat. Brush pan with canola oil. Once pan is hot, add steak and cook for 2-3 minutes. Flip steak and cook for another 2-3 minutes, or until steak is cooked to your preference.

Remove steak from pan and slice into 1-inch strips (Tip: Slice the steak against the grain.) Set steak aside.

Once onions are very soft and fully caramelized, place ½ Tbsp of salted butter in a large fry pan or griddle. Heat over medium heat until butter has mostly melted. Place a large flour tortilla into the pan. Layer the following ingredients on one half of the tortilla: ¼ cup of mozzarella cheese, ¼ pound of sliced steak, ¼ cup of caramelized onions, ¼ cup of Mexican cheese, 2 Tbsp of crumbled gorgonzola cheese and 1 Tbsp of green onions. Fold the other half of the tortilla over onto the fillings.

Cook over medium heat until bottom tortilla is golden (2-3 minutes). Flip and continue cooking until second side is golden.

Transfer quesadilla to a cutting board and slice into wedges. Garnish with sliced green onions before serving.