Heat vegetable oil or olive oil in a skillet over high heat. Sprinkle both sides of the chicken with taco seasoning. Add the chicken to the skillet and saute over medium-high heat until done, about 4 minutes per side. Remove from the skillet, allow to cool slightly, and dice into cubes. Set aside. In the same skillet over medium-high heat, throw in the onions and peppers and cook until the veggies are golden brown, 3 to 4 minutes. Remove and set aside. Melt 1/2 tablespoon of the butter in a separate skillet or griddle over medium heat and lay a flour tortilla in the skillet. Build the quesadillas by laying a good amount of grated cheese on the bottom tortilla, and then arranging the chicken and cooked peppers on the cheese. Top with a little more grated cheese and top with a second tortilla. When the tortilla is golden on the first side, carefully flip the quesadilla to the other side, adding another 1/2 tablespoon butter to the skillet at the same time. Continue cooking until the second side is golden. Repeat with the remaining tortillas and fillings. Cut into wedges and serve with salsa, sour cream, jalapeno slices, and cilantro.