In a 12-inch non-stick skillet, wilt spinach over medium-high heat, tossing several times to cook evenly. This will take only 3-5 minutes. (Add 1 tablespoon water if spinach sticks to pan while cooking; otherwise, do not add water.) Remove spinach to a small bowl and set aside.
In same skillet (no need to wash), cook bacon and onion until tender, about 10 minutes, adding garlic during the last minute or so. Sprinkle in flour, salt and pepper, and toss with bacon-onion mixture. Slowly add milk, bringing it to a boil, stirring to scrape any of the mixture off the bottom of pan. Cook and stir until thickened, a couple of minutes total. Add spinach and mix well, heating thoroughly.