Clean and slice the mushrooms, dice the onion & garlic, and shred your cheese (if not using pre-shredded).
Heat olive oil and butter in a large skillet over medium-high heat until butter is melted, then swirl to coat the pan.
Add the onion and mushrooms and cook for about five minutes, stirring occasionally, or until mushrooms start to brown and release their liquid.
Stir in the minced garlic and seasonings and continue to cook & stir for another two or three minutes, or until mushrooms are browned and softened and most of the liquid has evaporated.
Remove the cooked mushrooms to a separate bowl.
Start assembling your quesadillas, three at at time: Set out three tortillas. Using ½ cup of shredded cheese across the three, sprinkle cheese on one half of each tortilla. Top that cheese with some of the mushroom mixture.
Fold each tortilla over its cheese & mushroom filling. Cook your quesadillas three at a time in the same skillet you used for the mushrooms, over medium-high heat for one minute a side (or until nicely browned).
Repeat this process with the remaining tortillas, cheese, and mushrooms, so that you end up with twelve quesadillas across four batches. Add another bit of olive oil to the skillet in between each batch, as necessary.
Cut each quesadilla in half. Serve the mushroom quesadillas with any favorite add-ons, such as: Sour cream, guacamole, cilantro, salsa…
Notes
I don’t bother to wipe out the skillet after making the mushrooms, because I think that imparts a nice flavor and browning to the quesadillas. You can, though, if desired. Today I used all white mushrooms because that’s what I had on hand, but baby bellas are also very nice in these.