Season the chicken with 1 teaspoon salt, heat the oil in a skillet over medium-high heat and cook for 5 to 7 minutes per side until it reaches 165°F; rest and slice it.
Steam the broccoli for 4 minutes until bright green and barely tender, then spread it in a greased 9-by-13-inch baking dish with the chicken.
Melt the butter in a saucepan, stir in the flour for 1 minute and whisk in the milk, mustard and remaining salt until thickened.
Remove the pan from the heat, stir in 1 cup cheddar and the lemon juice and pour the sauce over the chicken and broccoli.
Mix the remaining cheddar with the panko, scatter it on top and bake at 400°F for 15 minutes until browned and bubbling.