How to Make It
Preheat oven to 400°F. Stir together butter, mustard, and 1/2 teaspoon each of the salt and black pepper in a medium bowl. Stir together panko, cornflakes, and Parmesan in a second shallow bowl. Dip chicken in butter mixture; dredge in panko mixture, pressing to adhere. Place on a baking sheet lined with aluminum foil. Bake in preheated oven until chicken is browned and done, about 17 minutes.
Meanwhile, stir together oil, garlic, crushed red pepper, and remaining 1 teaspoon salt and 1/2 teaspoon black pepper in a large bowl. Add broccoli, and toss to coat. Transfer to a rimmed baking sheet, and roast at 400°F until broccoli is browned and tender, about 17 minutes.
Melt butter in a 3-quart saucepan over medium-high. Whisk in all-purpose flour; cook, whisking constantly, 1 minute. Gradually whisk in whole milk. Bring to a boil, and cook, whisking constantly, until thickened, for 1 to 2 minutes. Whisk in grated Parmesan cheese, fresh lemon juice, salt, and black pepper. Serve immediately.