Place onions, butter and salt in large slow cooker. Cook on low for 8-10 hours. This works best if you do it overnight.
Heat oven to 350 F. Place ovenproof bowls onto baking sheet. Ladle soup into bowls. Top each bowl with 1-2 slices of bread and about ⅓ cup of shredded cheese.
Bake for 20 minutes.
Turn on broiler and broil until golden brown, keeping a very close eye on it.
Garnish with additional thyme leaves and enjoy!