This rich side takes its flavor cues from green bean casserole. Plus, who doesn’t want stuffing croutons?
In a medium saucepan, cook the shallots and garlic in 4 tbsp. butter over medium heat, stirring often, until tender, about 5 minutes. Add the cream and rosemary. Bring to a boil over high heat. Cook over medium until the liquid is reduced to 21/4 cups, about 15 minutes. Discard the rosemary sprigs. Season with salt and pepper.
In a large cast-iron skillet, heat 2 tbsp. oil over medium-high. Add half the Brussels sprouts, cut-side down; season. Cook until browned, about 3 minutes. Transfer to a large bowl. Repeat with the remaining 2 tbsp. oil and the remaining Brussels sprouts. Return all the Brussels sprouts to the skillet. Add about 1/4 cup water. Cover and cook over medium-high heat until almost tender, 3 to 4 minutes. Stir in the shallot mixture; season. Transfer to a 9-by-13-inch glass baking dish and arrange the Brussels sprouts in an even layer, cut-side up.
For the stuffing croutons, in a food processor, pulse the bread cubes until coarse crumbs form. In a medium bowl, toss the breadcrumbs with the remaining 4 tbsp. melted butter, the sage, poultry seasoning, and celery salt (if using); season generously.
Preheat the broiler. Sprinkle the stuffing croutons on top of the Brussels sprouts. Broil the Brussels sprouts until the croutons are golden brown and toasted, about 5 minutes.