Step 1Steam Brussels sprouts in a covered steamer basket over boiling water until tender, about 15 minutes.
Step 2Coat large saucepan with nonstick cooking spray and saute shallots over medium-low heat until tender, 2 to 3 minutes. Stir in milk and 1 cup broth. Bring to a simmer. Blend remaining 1/2 cup broth into flour until smooth, then whisk into milk mixture. Return to a simmer, stirring until smooth. Boil 1 minute. Stir in salt and white pepper to taste.
Step 3Add Brussels sprouts to sauce, stirring to coat, then spoon into lightly buttered 8-inch square baking dish. Combine bread crumbs, hazelnuts and thyme. Stir in melted butter. Sprinkle over top of vegetables. Bake at 400 degrees until hot and bubbly and crumbs are golden brown, 15 minutes.