In a skillet over medium heat, melt 1 tablespoon of butter. Add the sliced red onion and chopped sage. Cook, stirring occasionally, until the onions are soft and golden brown, about 20 minutes.
While the onions are caramelizing, butter one side of each slice of sourdough bread with the remaining tablespoon of butter.
Place one slice of bread, buttered side down, on a clean surface. Layer with smoked gouda slices, followed by roast beef, and then the caramelized onions. Top with the second slice of bread, buttered side up.
Heat a skillet over medium heat. Place the assembled sandwich in the skillet and cook until the bread is golden brown and the cheese is melted, about 3 minutes per side.