The butter with chopped onions put in a plastic bowl, quickly replace the lid and stew 2 minutes at 600 Deut. While stirring continuously, add the flour, broth and tomato paste. Sprinkled with salt and pepper, capped and kept for 3 minutes at 600 W, with stirring two or three times. The sauce can be seasoned with red wine, garlic, lovage, basil or rosemary.