Step 1

Toss butter, onions and shallots together in slow cooker. Cook on low for 8 to 10 hours. (I do this step overnight.)

Step 2

Heat butter in a large heavy bottomed pot over medium-high heat. Add onions and shallots, cooking until they become translucent, about 10 minutes.

Step 3

Cook, stirring every few minutes or so, until onions are browned and caramelized, about 30 to 40 minutes. Transfer caramelized onions to slow cooker.

Step 4

In slow cooker, stir together caramelized onions, garlic, sherry, beef stock, Worcestershire, thyme, salt, pepper and bay leaf.

Step 5

Cook on high for 4 to 6 hours or low for 8 to 10 hours.

Step 6

Before serving, set oven rack about 8 inches from broiler and heat the oven's broiler. Arrange bread slices on a baking sheet.

Step 7

Broil bread slices until toasted, approximately 1 to 2 minutes on each side. Remove from oven (leave oven on) and set aside.

Step 8

Fill oven-safe soup crocks 3/4 full of onion soup and place a slice of toasted bread on top of each bowl. Top each bowl with a slice of cheese.

Step 9

Place filled bowls onto a baking sheet and broil until cheese topping is lightly browned and bubbling, about 2 minutes. Serve immediately.