- In a non-stick skillet, caramelize the onions in the butter, about 20 minutes. Season with salt and pepper. Set aside.
- With the rack in the middle position, preheat the oven to 180 °C (350 °F).
- In a pot of boiling water, cook the sausage for about 5 minutes. Drain. Warm the bread in the oven.
- Preheat the oven’s broiler.
- Open the buns horizontally and spread mustard inside. Cover with caramelized onions and add the sausages. Sprinkle with cheese. Place the hot dogs on a baking sheet. Melt the cheese in the oven. Garnish with arugula.