1. Melt the 2 tablespoons butter in a 10-inch skillet over medium-high heat. Mound the onion in the skillet and add the salt and pepper. Reduce the heat to medium low and cook, stirring as needed, for 10 minutes, or until the onion is softened and lightly caramelized around the edges. Add the beer and cook for 25 minutes longer, or until most of the liquid has evaporated and the onion is very soft and flavorful. Remove the skillet from the heat and allow the onion to cool slightly.
2. Cut the smoked sausage in lengths to fit the bread; then halve the sausage pieces lengthwise. Oil a grill pan with the vegetable oil and heat it over high heat. Add the sausages and grill until the casings are blistered and crisp and the sausages are heated through. Transfer the sausages to a plate and immediately lay the cheese slices on top, so that the cheese softens.
3. Butter the insides of the bread, using 1 tablespoon of the softened butter for each sandwich. In batches, add the bread to the grill pan, buttered side down, and cook until crisp, about 2 minutes. Transfer to a plate and spread mayonnaise on one side and mustard on the other. Add the sausages and cheese, mound with the onion, and garnish with pickles. Serve immediately.
Yield: 4 sandwiches