Preheat the oven to 425°F (218°C). Line a baking sheet with aluminum foil and arrange the lemon slices in a single layer on the sheet.
Place the cut chicken pieces, trimmed and halved green beans, and sliced red onion in a large mixing bowl.
Drizzle the olive oil over the chicken and vegetables, then sprinkle the lemon pepper, kosher salt, and black pepper. Toss until everything is evenly coated.
Spread the chicken and vegetables in a single layer over the lemon slices on the prepared baking sheet.
Bake for about 15 minutes, tossing the chicken and vegetables once halfway through (leave the lemon slices in place) until the chicken is cooked through and the vegetables are slightly golden.
Remove from the oven, optionally garnish with fresh parsley and finish with Parmesan, extra garlic, soy sauce, or chili garlic sauce if desired, then serve warm.