In a large saute pan, melt the bacon grease over medium heat. Add the onion and garlic and stir to cook until the onions are very soft, about 5 minutes. Add the vinegar, crushed red pepper flakes, salt and pepper and stir; add the spinach and toss it around, cooking until it's just wilted, 1 to 2 minutes. Serve immediately so the spinach is wilted but not soggy.