Burger Dog

Bahan-bahan (4 porsi)

• 2 medium onions (sliced)
• 2 tbsp neutral oil
• 1 lb ground beef (lean preferred)
• 4 slices cheese (of choice, eg: gruyere or provolone)
• 2 tbsp butter (room temp)
• 4 hot dog buns
• whole grain mustard (to taste)