Ham and cheese bread pudding

4 tablespoons unsalted butter, plus 

  more for greasing pan

2 yellow onions, diced

2 tablespoons fresh thyme leaves, 

  plus more for garnish

2 tablespoons Dijon mustard

Kosher salt

Freshly cracked black pepper

8 large eggs

2 cups half-and-half

2 cups grated Gruyère cheese

6 ounces smoked ham, cut into    1-inch pieces

1 pound croissants, ripped into               2-inch pieces (about 10 cups)

1/2 cup grated Parmesan cheese

Flaky salt

Generously butter a 9-by-13-inch baking dish; set aside. In a skillet over medium heat, melt the butter. Add onions; reduce heat to low and cook until onions are very soft and beginning to caramelize, about 10 minutes. Add thyme, mustard, salt and pepper and continue to cook, stirring occasionally, for five minutes more. Remove from heat and cool slightly. In a large bowl, whisk eggs and half-and-half until combined; season with salt and pepper. Add onion mixture, Gruyère and ham; add croissant pieces and stir to coat. Transfer bread pudding mixture to the prepared dish, pressing gently. Top with the Parmesan and cover with aluminum foil. Transfer to the refrigerator to chill for at least one hour or up to overnight. When ready to prepare, preheat the oven to 350 degrees. Bake covered bread pudding on center oven rack for 30 minutes. Uncover and continue to bake until the Parmesan has melted and turned golden, about 20 minutes. Allow to cool for 10 minutes; serve warm with freshly cracked black pepper, flaky salt and tons of fresh thyme.