Melt the butter in your skillet, add the onion, garlic, and spinach along with a big pinch of sea salt. Cook over medium heat for about 2-3 minutes, until the spinach has wilted.Set the spinach mixture aside and add another tablespoon of butter, the beans, and salsa and cook over medium high heat for about 4-5 minutes, until the beans soften. You can press the back of your wooden spoon on the beans to squish them up a bit, making them more like refried beans. If the mixture gets dry just add a splash of water.Spread the spinach mixture over a tortilla, then the bean mixture, and then sprinkle with cheese.Spread a thin layer of butter over one side of the other tortilla and place it butter side up on top of the shredded cheese. The butter on the tortilla will help it to crisp and brown nicely.Place the prepared quesadilla on a wire rack, set on a baking sheet, and put under the broiler in the oven. Broil the quesadilla(s) on high for 4 minutes or until the top is browned and crisp. Take a peek after 2 minutes in case your broiler runs hotter than mine.Cut the quesadilla(s) into triangles and serve with sour cream, guacamole, or salsa! Garnish with chopped cilantro, chives, or parsley.