Melt 2 tablespoons butter in heavy large saucepan over medium high heat. Add onion and saute until tender and golden brown, about 15 minutes. Add garlic, cumin, cinnamon and allspice and stir 1 minute. Add beer, mustard and honey and bring to boil (sauce will foam). Remove from heat. Puree in batches in blender until smooth. Cool to room temperature. Pour into baking dish. Add pork loin; turn to coat. Cover and refrigerate 8 hours or overnight, turning occasionally.
Preheat oven to 375 degrees F. Remove pork from marinade; pat dry. Reserve marinade. Heat oil in heavy large skillet over medium heat. Season pork with salt and pepper. Add to skillet and brown on all sides. Transfer to baking sheet with rim. Roast in oven for 3/4 to 1 hour or until thermometer inserted into thickest part registers 155 degrees F. Transfer pork to work surface; reserve any pan juices.
Combine pan juices and marinade in saucepan. Bring to boil. Reduce heat to simmer. Mix butter and flour in small bowl until smooth paste forms. Add to sauce and simmer until sauce thickens. Season to taste with salt and pepper. Cut pork into slices. Pour sauce over.
The first Oktoberfest marked Prince Ludwig's marriage to Princess Therese; it was so much fun that it evolved into an annual celebration of German food and culture. Mexico has a long tradition of producing beer. It is said to have had the first commercial brewery in the New World (in the 1500s) and has been making lagers since 1880s or 1890s. Corona still enjoys a bit of a following in the U.S. which is funny because it is made as cheaply as possible so it can be sold inexpensively to manual workers in Mexico. Every Mexican brewery has a product in its portfolio. Some of the best Mexican beers are the chocolatety Negro Modelo, which is perfect with chicken mole, the dark reddish amber Dos Equis, and Nochebuena. In Mexico, beer is still sometimes served at room temperature which is especially good for the heartier beers. Lighter bodied beers are better served chilled.