Step 1

Place an oven rack in the lower-middle position.

Step 2

Preheat the oven to 400 degrees F.

Step 3

Generously grease a 9x13-inch baking dish and the inside of a large Dutch oven with nonstick cooking spray.

Step 4

Place the baguette slices onto a metal rack that's resting on a baking sheet.

Step 5

Bake the bread slices until golden brown, about 8-10 minutes.

Step 6

Transfer the bread from the oven and set aside.

Step 7

Add the sliced Vidalia onions, the sliced yellow onions, the butter, the black pepper, the salt, the bay leaf, and the thyme to the prepared Dutch oven.

Step 8

Cover the Dutch oven tightly and cook in the oven for 1 hour.

Step 9

Transfer the pot from the oven, then stir and scrape the bottom and sides of the pot.

Step 10

Place the pot back into the oven with the lid slightly off and continue cooking until the onions are tender and slightly darker than golden-brown, about 1 hour-1 hour 30 minutes.

Step 11

Transfer the pot to the stovetop over medium-high heat, stirring and scraping the sides and bottom. Use caution as the pot will be hot.

Step 12

Remove and discard the bay leaf from the onions.

Step 13

Add in the sherry, the chicken broth, and the beef broth.

Step 14

Bring the mixture to a boil, stirring frequently, until slightly thickened, about 4-5 minutes.

Step 15

Layer 1/2 of the baked baguette slices into the prepared casserole dish.

Step 16

Spoon the onion mixture evenly over the bread.

Step 17

Top the onion layer with the remaining bread slices.

Step 18

Sprinkle the shredded cheese evenly over the top of the casserole.

Step 19

Cover the casserole with foil and bake for 30 minutes.

Step 20

Set the oven to broil and remove the foil.

Step 21

Broil until the cheese is bubbly, about 3 minutes. Keep a close eye on it so it does not burn.

Step 22

Garnish with extra thyme leaves and serve.