Steps
1. Heat butter in a pan over medium heat.
2. Add sliced onions and cook until caramelized, about 8 minutes.
3. In a small bowl, mix shredded cheese, milk, mustard, salt, and pepper.
4. Toast the bread slices lightly.
5. Spread the cheesy mustard mixture evenly on each slice.
6. Top with caramelized onions.
7. Place the slices under a broiler or in a hot oven for 3-5 minutes until cheese melts and bubbles.
8. Serve warm.