1 Preheat the oven to 400 F.
2 In a deep bowl, mix the onions, lemon juice, oil, mustard, garlic, salt and a liberal grinding of pepper.
3 Mix them well.
4 Add the short ribs and turn them with a spoon to coat evenly.
5 Allow the ribs to marinate at room temperature for about 2 hours, turning them from time to time.
6 On a rack set in a shallow roasting pan, arrange the ribs fat side up in a single layer
7 Roast the ribs uncovered in the middle of the oven for 20 minutes.
8 Lower the heat to 350° and continue to roast for 1 hour and 15 minutes longer.
9 On a heated serving platter, arrange the ribs and serve at once.