Preheat oven to 450°F (232°C). Line a baking sheet with parchment or leave unlined. Arrange the 5 to 7 large croissants, cut-side up, on the sheet and toast in the oven 5 to 8 minutes, until lightly browned and toasty. Remove and let cool slightly, then chop or tear into roughly 1–2-inch pieces.

Measure the 2 tablespoons unsalted butter and split it: use 1 tablespoon for cooking and reserve the other tablespoon for brushing the baking dish.

Heat a large skillet over medium-low heat and add the 1 tablespoon butter. Add the diced sweet onion and cook, stirring occasionally, about 5 minutes until softened. Add the minced garlic and cook 30–60 seconds until fragrant.

Add the chopped cremini mushrooms to the skillet and cook about 5 minutes more, stirring occasionally, until the mushrooms release their liquid and become tender.

Add the fresh baby spinach to the skillet and stir until wilted. Season the vegetable mixture with kosher salt and freshly ground black pepper to taste. Remove from heat and set aside.

Melt the reserved 1 tablespoon butter (if it is solid) and use it to brush a 9x13-inch baking dish, coating the bottom and sides.

Place the toasted croissant pieces in the prepared 9x13 baking dish. Evenly spread the spinach–mushroom–onion mixture over the croissants and toss gently in the dish so the pieces are well distributed.

In a large bowl, whisk together the 8 large eggs, 2 1/2 cups milk, 1 cup heavy cream, and a pinch of kosher salt and pepper until blended.

Stir approximately half of the freshly grated 8 ounces of gruyere into the egg mixture (reserve the remaining gruyere for the top). Pour the egg-and-cheese mixture evenly over the croissants in the baking dish, pressing any large croissant pieces down so they absorb the liquid.

Cover the dish tightly with plastic wrap and refrigerate for at least 8 hours or overnight to allow the croissants to soak up the custard.

When ready to bake, preheat the oven to 350°F (177°C). Remove the baking dish from the refrigerator and let it sit at room temperature for 30 minutes while the oven heats.

Remove the plastic wrap, sprinkle the reserved gruyere evenly over the top, and bake uncovered at 350°F for about 45 minutes, until the top is golden and the center is set (a knife inserted near the center should come out mostly clean).

Remove from the oven and let rest a few minutes. Top with grated parmesan cheese and fresh chopped parsley to taste. Serve warm.