Preheat the oven to 400°F.

Toss fennel, 2 teaspoons of the oil and 1/4 teaspoon of the salt on a large rimmed baking sheet.

Toss asparagus, remaining 2 teaspoons oil and 1/4 teaspoon salt on a separate large rimmed baking sheet.

Roast vegetables until golden and tender, about 10 minutes.

Transfer fennel and asparagus to a 9 1/2- or 10-inch pie plate and stir together. Set aside.

Melt butter in a medium saucepan over medium-high heat. Whisk in flour and cook, whisking constantly, for 1 minute.

Gradually add milk and bring to a boil.

Reduce heat and simmer, whisking frequently, until the mixture is beginning to thicken, about 3 minutes.

Remove from heat and stir in pepper, nutmeg and 1/4 cup of the cheese.

Pour mixture over roasted vegetables.

In a small bowl, stir together bread crumbs and remaining 1/4 cup cheese.

Sprinkle mixture over vegetables.

Bake until heated through and bread crumbs are golden, 20 to 25 minutes.