Onion and Cider Soup Gratin

1/2 cup (1 stick) butter
5 cups thinly sliced yellow onion
4 cups hard cider
4 cups strong vegetable stock
freshly ground black pepper
1 French baguette, sliced about 1/2 inch thick
2 1/2 cups freshly grated firm smooth cheese such as cheddar, Cantal, Emmentaler or Gouda (about 8 oz)

In stock pot, melt 1/4 cup of butter over low heat
Add onion, cover and cook stirring occasionally until onion begins to color, about 25 minutes
Uncover and increase heat to medium and cook until onion is deep golden and almost caramelized, about 25 minutes longer
Add cider and stock to the caramelized onion and bring to a boil over medium high heat, season to taste with salt and pepper
Reduce heat to low and simmer, uncovered, for 30 minutes
Pre-heat oven to 325 degrees
Melt remaining 1/4 cup butter and brush both sides of bread, arrange in single layer on baking sheet and bake until crisp and golden, about 25 minutes
Place 3 slices of toasted bread in the bottom of each crock, ladle soup onto crocks and sprinkle a portion of cheese on top of each serving
Broil until cheese is brown and bubbly, about 2-3 minutes

Must be served/eaten immediately