Combine cheese in a medium bowl, set aside.
Heat a large skillet over medium-high heat. Melt one tablespoon of butter in skillet and sauté bell pepper, mushrooms and onions until tender in butter. Remove from skillet.
Butter one side of a tortilla and place, butter-side down, in skillet. On one half of tortilla, sprinkle a layer of the combined cheeses. Top the cheeses with ¼ cup of spinach, ¼ of the veggie mixture and finally another layer of cheese.
Using a spatula, fold the empty half of the tortilla over on top of the ingredients. Allow to cook until the bottom side is golden brown.
Flip and cook on other side until it is golden brown and the cheese is fully melted.
Repeat with remaining tortillas and ingredients.
Cut each quesadilla into 4 triangle-shaped wedges and serve with guacamole, salsa and sour cream for dipping.