For the jam:
2 tablespoons of salted butter
1 medium onion (about 1/2 pound) thinly sliced
1/4 teaspoon salt
1 tablespoon packed light brown sugar
2 ounces blue cheese
2 tablespoons salted butter
8 slices sourdough bread (1/4 inch thick)
1/4 pound cooked roast beef, thinly sliced
6 ounces sharp cheddar cheese, coarsely grated
To make the jam: in a large nonstick skillet, melt the salted butter over medium-high heat. Add the onions and cook, stirring occasionally, until they are lighly golden, about 6 minutes. Reduce heat to low and cook, stirring occasionally, until the onions are a deep golden color, 8 to 10 minutes. Add the salt and brown sugar, and stir until the sugar has dissolved and the onions look glazed, about 1 minute. Remove the onions from the pan and let cool. (The jam can be prepared up to 6 hours in advance and kept covered at room temperature.)
In a small bowl mash the butter and blue cheese together.
To assemble: Spread the blue cheese butter on one side of each slice of bread. Place 4 slices on your work surface, buttered side down. Distribute the roast beef evenly over the 4 slices, folowed by the cheddar cheese. Spoon on the jam, and top with the remaining 4 bread slices, buttered side up.
Make on the stove top or in a sandwich maker.