Heat a large skillet with a few sprays of cooking spray over medium-high heat. Sauté the pepper, onion and mushrooms until tender crisp, about 4-5 minutes.
Spread ½ Tbsp. butter onto one side of each tortilla.
Heat a skillet to medium and place a tortilla, butter side down, in the skillet. Sprinkle ¼ cup cheese and a ¼ cup mushroom mixture on top. Place the 2nd tortilla, butter side up, on top.
Cook for 2-3 minutes per side, or until golden brown. Repeat with the remaining quesadillas.
Slice each quesadilla into four slices and serve with guacamole or sour cream.
Tip: allow the quesadillas to cool slightly and use a pizza cutter to quickly cut into four slices.