Preheat the oven to 350°F. With a small paring knife, make shallow, diagonal cuts in the sausages. Place them in a shallow baking pan and bake for about 5 minutes, until brown. Remove from the oven and reserve. Heat the butter in a large skillet over high heat, add the onions, shallots, and garlic, and cook for 3 minutes, until golden. Add the sausages. Add the ale to deglaze the pan, loosening any browned bits on the bottom of the pan with a wooden spoon. Cook over medium heat for about 10 minutes, until the ale is reduced so it just coats the bottom of the pan. Transfer the sausage to a serving platter and keep warm. Increase the heat to high and stir the demi-glace and mustard. Cook for 5 to 8 minutes, until the mixture is reduced by half. Season with salt and pepper to taste. To serve, spoon the onion mixture over the sausages.
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