Preheat oven to 350 degrees F. Cook long grain and wild rice mix according to package directions.
Meanwhile, trim fennel, reserving some of the feathery leaves. Cut bulb into thin slivers. In a very large skillet cook fennel slivers, sausage, carrots, and onion over medium heat for 5 to 10 minutes or until sausage is browned and vegetables are crisp-tender, stirring occasionally. Stir in garlic.
In a medium bowl combine flour, sour cream, and mustard; gradually stir in milk. Add milk mixture all at once to sausage mixture. Cook and stir over medium heat until slightly thickened and bubbly. Stir in asparagus and cooked rice. Transfer mixture to a 2 1/2- to 3-quart casserole.
Bake, covered, for 20 to 25 minutes or until heated through. Top with the reserved fennel leaves.