Crockpot Caramelized Onion Crostini

I can't believe I've never posted this recipe! It's one of my favorite appetizers. It comes from 125 best vegetarian slow cooker recipes. I make smaller portions usually.
Ingredients:

• 3 lbs onions
• 3 tbsp melted butter
• 1 tbsp granulated sugar
• 1 tsp salt
  
• 1 tsp cracked black peppercorns
• 1 tbsp fresh thyme leaves
• 1 tsp balsamic vinegar
• 16 crostini
• 2 cups shredded Swiss or Gruyere cheese
  

Procedure:

1. Slice onions really thin.
   
2. Cook onions and melted butter in crockpot for 30-60 minutes until onions are softened.
   
3. Add sugar, salt, and peppercorns. Put a towel or two under the crockpot lid to absorb moisture.
   
4. Cook on high for 4 hours, stirring a few times.
   
5. Stir in thyme and balsamic vinegar.
   
6. Preheat broiler.
7. Spread onions evenly over crostini and sprinkle cheese on top. Place on baking sheet and broil until cheese is melty - about 3 minutes. 5 results in charcoal.
   

I like to use more crostini and less onions on each one, and use a full loaf of french bread. I've also used this to stuff tomatoes (it wasn't very good) or to make a caramelized onion filling for sandwiches, or just to have caramelized onions at dinner. I love that I don't really have to babysit the crockpot, although I do feel I can't leave the house for a super-long period of time.