Prepare the Brussels sprouts: Trim the tough ends, remove any outer yellowed leaves, and halve them lengthwise. Quarter large ones for even cooking. Set aside.

Cook the bacon: In a large skillet, cook the diced thick-cut bacon over medium heat until it's beautifully crispy and golden brown. Transfer to a paper towel-lined plate, leaving about 1 tablespoon of bacon fat in the skillet.

Sauté aromatics: Add olive oil to the skillet with the reserved bacon fat (add more olive oil if needed). Add chopped onion and cook until softened and translucent, about 5–7 minutes. Stir in minced garlic and cook for another minute until fragrant. Scrape into a large mixing bowl.

Cook the Brussels sprouts: Add prepared Brussels sprouts to the same skillet (no need to clean) and sauté over medium-high heat for 8–10 minutes until tender-crisp and caramelized. Add to the bowl with onions and garlic.

Combine with bread and bacon: Add the cubed stale bread to the bowl. Scatter 1 cup of grated Gruyère cheese and most of the crispy bacon bits over the mixture and toss until evenly distributed.

Prepare the custard: In a separate bowl, whisk together eggs, whole milk, heavy cream, Dijon mustard, salt, pepper, and a pinch of nutmeg until smooth.

Assemble the bread pudding: Pour custard mixture over the bread and vegetable mixture in the large bowl. Gently fold everything together until all bread cubes are moistened. Press down slightly to help absorb. Let sit for 20 minutes (or cover and refrigerate up to 12 hours) to soak.

Preheat oven and prepare dish: Preheat oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.

Bake: Pour the mixture into the prepared baking dish, spreading evenly. Sprinkle remaining ½ cup of grated cheese on top. Bake for 35–45 minutes, or until golden brown, custard is set, and edges are bubbling. Tent with aluminum foil if top browns too quickly.

Rest and serve: Cool for 10 minutes before serving. Garnish with fresh parsley and reserved bacon bits if desired.