Heat the butter in a large heavy-bottomed nonreactive saucepan over medium-high heat. Add the onions and cook for 30 minutes or until they begin to caramelize, stirring occasionally. Add the sugar, stir well to combine and cook for 3 minutes. Deglaze the pan with the beer and vinegar, scraping the bottom to loosen any browned bits. Cook, stirring occasionally, for about 8 minutes or until the liquid has reduced by half. Add the stock and stir well to combine. Bring the mixture to a boil and cook for 20 minutes or until the liquid has again reduced by half, stirring occasionally. Stir in the thyme. Remove the saucepan from the heat. Season to taste with salt and pepper. When the mixture is cool enough to handle, puree it, by batches if necessary. Be careful not to over process the sauce. It should be the consistency of loose applesauce. Adjust the consistency, returning it to the heat if it needs thickening or adding more stock to thin it. Tightly covered, the sauce will keep in the refrigerator for a week. Makes 3 1/2 cups.

Combine first six ingredients. Place roast in a large clear plastic bag. Pour in marinade and close bag tightly. Marinate 2-3 hours at room temperature or overnight in refrigerator. Remove meat from marinade. Place on rack in shallow roasting pan. Roast uncovered at 325 degrees for 3 hours or until meat thermometer registers 175 degrees. Baste occasionally with marinade during last hour of roasting time. Serves 8-10 and is great with the caramelized red onion sauce.