Preheat oven to 400°.
Melt butter in a large nonstick skillet over medium heat. Add onion and garlic to pan; cook 4 minutes, stirring occasionally. Stir in flour; cook 2 minutes, stirring frequently. Gradually add milk, stirring constantly with a whisk; cook 8 minutes or until mixture thickens, stirring occasionally. Add cream cheese, mustard, salt, and freshly ground black pepper; cook 1 minute or until cheese melts. Stir in rice and Brussels sprouts. Spoon rice mixture into an 11 x 7-inch baking dish coated with cooking spray. Sprinkle with Parmesan cheese and chopped pecans. Bake at 400° for 20 minutes or until bubbly.