Preheat oven to 400 degrees. In a medium bowl, stir together butter, mustard, salt and pepper. Combine panko, cornflakes, and Parmesan in a second shallow bowl. Dip chicken in butter mixture; dredge in panko or breadcrumbs mixture, pressing to adhere. Place on a baking sheet lined with aluminum foil. Prepare broccoli and then bake chicken and broccoli in separate pans at the same time for about 17 minutes or until chicken is crispy and cooked to 165 degrees.
For the Roasted Broccoli:
Stir together oil, garlic, red pepper, salt and pepper in a large bowl. Add broccoli, and toss to coat. Transfer to a rimmed baking sheet, and roast at 400 degrees until broccoli is browned and tender, about 17 minutes.
For the Creamy Parmesan Sauce:
Melt butter in a 3-quart saucepan over medium-high. Whisk in flour and cook until thickened. 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. (If too thick, add more milk.) Serve immediately with chicken.