French dip sandwich with beef and gravy

Ingredients:

• 3.5 tbsp unsalted butter
• 1 large onion, thinly sliced
• 1.5 cups lightly salted beef broth
• 1/4 teaspoon freshly grated nutmeg
• 1 tbsp horseradish + more to taste
• 220 g thin slices of Gruyere
• 350 g thin slices of roast beef
• 4 hoagie buns, split in half on the sides

Preparation:

1. In a large nonstick skillet, melt 1 tablespoon butter over medium-high heat. Add the onion and 1/4 teaspoon salt; cook, stirring, until golden brown, about 10 minutes. Transfer to a bowl; do not wash the skillet.
2. Meanwhile, prepare the sauce.:
   
   In a small saucepan, bring the beef broth to a boil, adding the nutmeg. Remove from heat and stir in the horseradish and 1/2 tbsp butter. Cover and keep warm.
3. Top the buns with half the cheese, then the roast beef, onions, and the remaining cheese. Melt 1 tablespoon butter in a skillet over medium heat. Add 2 sandwiches and press them down with a heavy skillet. Cook, pressing down on the skillet to flatten the sandwiches, until the bread is golden brown and the cheese is melted, about 4 minutes per side. Repeat with the remaining 1 tablespoon butter and 2 sandwiches. Serve with gravy.