Preheat the oven to 425°F (220°C). Line a baking sheet with aluminum foil for easier cleanup and arrange the lemon slices evenly across the sheet.
Place the cut chicken, trimmed and halved green beans, and sliced red onion on the baking sheet over and around the lemon slices in a single layer.
Drizzle the olive oil evenly over the chicken and vegetables, then sprinkle the lemon pepper seasoning, kosher salt, and black pepper over everything; toss gently to coat using tongs or a spatula.
Bake for about 15 minutes, then use tongs or a spatula to toss the chicken and vegetables (leave the lemon slices on the pan) for even browning.
Return to the oven and bake until the chicken is cooked through and vegetables show some golden color, about another 5 minutes; total baking time will vary with chicken thickness—check for doneness.
Remove from oven, garnish with fresh parsley if using, and finish with any optional ingredients (Parmesan, extra garlic, soy sauce, or chili garlic sauce) to taste.