Season chicken with salt and pepper. Heat 1 tablespoon butter in a large skillet and add chicken. Cook until chicken is white throughout, about 4-5 minutes. Transfer chicken to a medium bowl and set aside. Wipe skillet clean.
Melt 1 tablespoon butter in large skillet under medium high heat. Add onions and bell pepper and saute until starting to soften, about 5 minutes or so. Transfer to a medium bowl and set aside.
Without wiping pan, add tortilla. (You can add more butter if needed) Start assembling quesadillas by placing a layer of shredded cheese, onion/pepper mixture, tomatoes, chicken, salsa, ranch dressing, and another layer of cheese. Yeah, these things are loaded. And you can practically add as much or as little of everything that you want. Cook until tortilla is starting to brown and cheese is starting to melt. Carefully flip over (you may want to have a plate handy for this), and cook until other side is nice and golden brown and cheese is fully melted.
Cut into quarters with a pizza cutter and serve with salsa, sour cream, and ranch dressing on the side.