Heat oven to 400°F and set oven rack to lower-middle position and spray a 9x13-inch baking dish with nonstick cooking spray.
Place the baguette slices on a metal rack placed inside a baking sheet.
Bake 8-10 minutes or until golden brown. Remove from the oven and set aside until ready to use.
While the bread slices toast, generously spray the inside of a large Dutch oven with nonstick cooking spray. Slice the onions (if they haven’t already been sliced).
Add the butter, sliced onions, salt, black pepper, thyme leaves and bay leaf to the Dutch oven.
Once the baguette slices have toasted, cover the Dutch oven tightly and cook the onions 1 hour. Remove pot from oven and stir, being sure to scrape the sides and bottom of pot.
Return the pot to the oven with the lid slightly ajar and continue cooking another 1 hour. Once again, remove the pot from the oven and
stir the onions (scraping the sides and bottom). The onions will be soft and a little darker than golden brown. If the onions need more color, return the pot to the oven (still with the lid slightly ajar) and cook another 30 minutes.
Transfer the pot to the stovetop and turn the heat to medium-high. *Remember – the pot is HOT! Use oven mitts when touching the handles.*
Remove and discard the bay leaf from the onions.
Add sherry, beef broth and chicken broth; bring to a boil, stirring constantly, until slightly thickened, about 4-5 minutes.
Layer half the toasted baguette slices in the prepared casserole dish.
Spoon onion mixture evenly over the bread, and top the remaining bread slices.
Sprinkle with shredded cheese and cover with foil.
Bake at 400°F for 30 minutes.
Set the heat to broil and remove the foil.
Broil 3 minutes or until cheese is bubbly. Watch closely.
Garnish with additional thyme leaves, if desired.