Trim the ends off each onion then halve lengthwise. Remove peel and finely slice using v-slicer. Add butter to dutch oven over medium heat. Once butter has melted add a layer of onions and sprinkle with a little salt. Repeat layering onions and salt until all onions are in.
Add thyme and bay leaf (keep both whole to pull out at end)
Do not try stirring until onions have sweated down for 15 to 20 minutes. After that, stir occasionally until onions are dark mahogany and reduced to approximately 2 cups. This should take 45 minutes to 1 hour. Do not worry about burning until close to end.
Add enough wine to cover the onions and turn heat to high, reducing the wine to a syrup consistency.
Add beef and chicken broth, and apple cider. Reduce heat and simmer 15 to 20 minutes.
Place oven rack in top 1/3 of oven and heat broiler.
Cut bread at an angle in rounds large enough to fit mouth of oven safe soup crocks. Place the slices on a baking sheet and place under broiler for 1 minute.
Season soup mixture with salt and pepper.
Remove thyme and bay leaf and ladle soup into crocks leaving one inch to the lip.
Place bread round, toasted side down, on top of soup and top with grated cheese. Broil until cheese is bubbly and golden, 1 to 2 minutes.