Season chicken with 1 teaspoon salt. Heat oil in a skillet over medium-high; cook 5 to 7 minutes per side until 165°F. Rest and slice.
Steam broccoli 4 minutes until bright green and barely tender. Spread with chicken in a greased 9-by-13-inch dish.
Melt butter in a saucepan; stir flour 1 minute. Whisk in milk, mustard, and remaining salt until thickened.
Remove from heat; add 1 cup cheddar and lemon juice. Pour over chicken and broccoli.
Mix remaining cheddar with panko; scatter on top. Bake at 400°F for 15 minutes until browned and bubbling.