Step 1

Preheat the oven to 350°F and lightly coat a 9 x 13-inch baking dish with cooking spray. Line a baking sheet with parchment paper.

Step 2

In a large saucepan, melt the butter over medium heat. Add the onions and cook, stirring occasionally, until the onions are soft and golden. Halfway through the cooking, around 2 minutes, add the sugar to help the onions caramelize. When the onions are done, add the chicken and beef broths, thyme, mustard, salt, and pepper. Simmer for 5 to 10 minutes.

Step 3

Place the bread slices on the parchment-lined baking sheet and toast 2 to 3 minutes on each side.

Step 4

Place half of the bread slices in the baking dish, overlapping slightly.

Step 5

Layer half of the onion mixture on top of the bread slices. Top with remaining bread slices, then the remaining onion mixture. Cover with the cheese. Cover with foil and bake until hot and bubbling, 30 to 35 minutes. Garnish with fresh thyme and serve.