Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Wash and dice the red potatoes into 1/2-inch cubes and trim the ends off the fresh green beans.
Cut the chicken breast into bite-sized pieces to ensure even cooking with the potatoes.
In a large mixing bowl, combine the chicken, potatoes, olive oil, rosemary, garlic powder, salt, and pepper, tossing until everything is well-coated.
Spread the chicken and potato mixture onto the prepared baking sheet in a single layer, leaving space for air to circulate.
Roast for 15 minutes, then remove the tray to add the green beans, tossing them with the existing juices on the pan.
Return to the oven for another 10 to 12 minutes until the chicken is cooked through and the potatoes are crispy and golden.
Finish the dish by squeezing fresh lemon juice over the tray to brighten the earthy herb flavors before serving.