To make the pesto: Add the arugula, garlic, cheese, and walnuts to the bowl of a food processor or blender, and puree. As the processer is running, drizzle in the olive oil. This will be used to garnish the Chorizo. Season the mixture with salt, and pepper, to taste. If the pesto is too thick, you can thin it out with water. To make the sausage: Cut the sausage link into large 1 1/2-inch chunks. Heat the oil in a saute pan, and saute the sausage until the edges start to brown, about 3 to 4 minutes. Toss the pieces while they fry so they cook all over. Add a dash of sherry vinegar to the pan to deglaze and release all the flavors. Transfer the sausage to a platter and serve with dollops of the arugula pesto. Add little cocktail sticks or toothpicks to each piece for easy eating.