In a shallow pan, heat the oil over low-medium heat. Slice your onions into thin wedges and add to the pan. Cook for 5-10 minutes, until the onions begin to caramelize.
Slice the mushrooms and add to pan, continuing to cook for another 10 minutes.
Chop zucchini into matchsticks and add to pan, cooking just until soft. Add balsamic vinegar and coat all vegetables, Remove from pan and set aside.
Wipe out any remaining balsamic (the sugars will burn). Place tortilla in pan (add a little more oil if your pan needs it) for 30 seconds or so, then flip and add half of the blue cheese. Fill half tortilla with vegetables, top with remaining blue cheese, and fold over. Cook until warm and gooey, flipping once.
NOTE: You can also bake quesadillas in oven for 5-10 minutes if you’d prefer.