Heat a griddle over medium heat. Melt a tablespoon of butter on it. Add the onions & sage, stirring to coat with the butter. Reduce the heat to medium low and let the onions saute, stirring every now and then, until they're nice and caramelized. This could take anywhere between 20-30 minutes.
1 large white onion, 2 tbsp butter, 1/8 tsp dried sage
Butter the outsides of both pieces of bread with the remaining tablespoon of butter.
2 slices sourdough bread
Raise the heat to medium, add one slice of bread buttered side down to the center. If using, spread half the horseradish evenly out over the inside of the slice of bread.
1-2 tbsp creamy horseradish
Top with 2 slices of cheese. Pile on the meat in an even layer. Top with a generous helping of the caramelized onions. Top with the rest of the cheese. Spread the inside of the remaining slice of bread with the rest of the horseradish, and place it on the sandwich, buttered side up.
4 slices smoked Gouda cheese, 1/4 lb sliced quality roast beef
Let the sandwich sizzle, using a flat spatula to press it down once or twice to flatten some, until the bottom is golden brown and the cheese is melted, about 1-2 minutes.
Carefully use the spatula to flip the sandwich and cook until the other side is also golden brown and cheese is melted, about another 1-2 minutes.
Transfer the sandwich to a cutting board. Rest 30 seconds before slicing & serving. We like ours with either chips, but a warm bowl of soup would be yummy too!