Cut onions in half, and slice thinly. Place them in the slow cooker. Add melted butter and stir well so all onions are coated. Put this on low overnight, 10 - 12 hours.
Add the garlic. Stir sherry into onion mixture; scrape the bottom to dissolve the small bits of browned food.
Throw in the thyme, bay leaf, salt/pepper, and broth. Stir well. Cook on low for 8 - 10 hours or on High for 4 - 6 hours.
Preheat oven to 400ºF. Slice bread to ½" thick slices. Brush both sides lightly with olive oil. Arrange on a baking sheet and bake 6 - 8 minutes until golden brown at the edges.
Once soup is out, set oven to 'broil'.
Pour soup into bowls ¾ full or so. Sprinkle the top with a bit of your cheese. Lay the bread on top and sprinkle the rest of the cheese on top. Broil for 2-3 minutes until cheese topping is lightly browned and bubbling.
