In a large saucepan stir together mustard, onion, honey and beer. Bring to a boil, stirring until marinade rises and foams. Remove from heat. In a blender puree marinade in 2 batches. Cool at room temperature and spoon off remaining foam.
- In a large re-sealable plastic bag combine pork and marinade and seal bag, pressing out any excess air. Put bag in a baking pan and marinate pork, chilled turning bag once or twice overnight.
- Let pork in marinade come back to room temperature about 40 minutes.
- Transfer marinade to a saucepan and bring to a boil
- Preheat oven to 375°.
- Pat pork dry with paper towels and season with salt and pepper. In a flame proof roasting pan heat oil over med high until hot but not smoking and brown pork on all sides.
- Roast pork in middle of oven, basting frequently with some marinade until thermometer registers 155° - 1 to 1 ½ hours.
- Transfer pork to a cutting board, saving juices in roasting pan and discard string. Let stand covered loosely with foil, about 15 min.
- While pork is resting, skim and discard fat from the pan and add remaining marinade. Deglaze the pan over med-high heat scraping up brown bits. Bring sauce to a boil and strain through a fine sieve into another saucepan. Bring sauce to a simmer and whisk in Beurre Manie, bit by bit whisking until sauce is combined and thickened slightly.