Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
Wash and cube the red potatoes into 1-inch pieces, and trim the woody ends off the asparagus spears.
Slice the chicken breast into even, bite-sized medallions to ensure they cook at the same rate as the potatoes.
In a large mixing bowl, whisk together the olive oil, lemon juice, minced rosemary, garlic powder, salt, and pepper.
Add the chicken and potatoes to the bowl, tossing thoroughly until every piece is coated in the herb oil.
Spread the chicken and potatoes in a single layer on the prepared baking sheet, nestling thin slices of the half lemon among them.
Roast for 15 minutes, then remove the pan to toss the asparagus spears with the remaining juices on the tray.
Return the pan to the oven for another 10 minutes until the chicken is opaque throughout and the potatoes are fork-tender and golden.