In a large bowl, whisk together the eggs, half and half, whole grain mustard, Dijon mustard, kosher salt, black pepper, and freshly grated nutmeg until smooth and well combined. Set aside at room temperature while you prepare the other components. This custard is the foundation of the bake, and whisking it early ensures all ingredients are fully incorporated before it needs to soak into the croissants.

Melt the unsalted butter in a large skillet over medium heat. Add the thinly sliced yellow onion and cook for 2-3 minutes until softened and translucent. Then add the fresh sage and fresh thyme, cooking for another 2-3 minutes to release the aromatic oils and infuse the butter with herbal flavor. I like to let the herbs bloom in the hot butter just long enough to smell their perfume—it transforms the whole dish. Remove from heat and let cool slightly.

Tear the day-old croissants into large bite-sized pieces and spread them on a baking sheet in a single layer. Place in a 250°F oven for about 40 minutes, stirring occasionally, until they're lightly toasted and slightly dried out. Day-old croissants are crucial here because they're sturdy enough to soak up the custard without falling apart, and the light toasting helps them hold their structure even better during the bake.

Grease a 9x13 inch baking pan with butter. Layer half of the toasted croissants from Step 3 on the bottom of the pan, then distribute half of the sliced ham and half of the shredded gruyere cheese over them. Scatter half of the caramelized onion mixture from Step 2 (including the herbed butter) over the cheese. Top with the remaining croissants, then the rest of the ham, cheese, and caramelized onions. This layering ensures even distribution of flavors and prevents the ham and cheese from settling to the bottom.

Pour the custard base from Step 1 evenly over the assembled layers, then gently press down on the croissants with the back of a spatula to help them absorb the liquid. Let the bake sit at room temperature for about 30 minutes, allowing the croissants to soak up the custard—this creates a creamy, cohesive texture rather than dry pockets. While soaking, preheat your oven to 350°F.

Bake the assembled bake in the preheated 350°F oven for 40-45 minutes, until the top is golden brown and a knife inserted into the center comes out clean with no uncooked custard. Let it rest for 5 minutes before serving. If desired, drizzle with hollandaise sauce and garnish with minced chives for extra richness and a fresh onion brightness that cuts through the richness of the ham and cheese.