Arrange a rack in the middle of the oven and heat the oven to 425°F. Lightly coat a baking dish large enough to fit the chicken in a single layer with some room around each one with butter.
Melt 3 tablespoons unsalted butter in a small saucepan over medium-low heat or in a small bowl in the microwave. Add 1 tablespoon salt-free Lemon Pepper seasoning and 1 teaspoon kosher salt and stir until combined.
Pat 4 boneless, skinless chicken breasts dry with paper towels. Place the chicken in the prepared baking dish, spaced slightly apart. Brush the chicken all over with all of the butter mixture.
Thinly slice 1 lemon and scatter the slices around the chicken. Bake until the chicken is golden-brown, cooked through, and registers 165°F on an instant-read thermometer, 25 to 35 minutes. Meanwhile, coarsely chop enough parsley leaves to get 2 tablespoons. Serve the chicken garnished with the parsley.