Step 1: Prep Potatoes and Chicken: Cut the potatoes into 2cm chunks (uniform size for even cooking). Pat the chicken breasts dry with paper towels — dry surfaces roast to a better crust instead of steaming in their own moisture.
Step 2: Toss in Olive Oil and Rosemary: In a large bowl, stir the olive oil, rosemary, and black pepper. Toss the potato chunks to coat (they need the longest, so oil them first). Brush the chicken on both sides with the same mixture.
Step 3: Arrange and Tuck in Lemon: Line the sheet pan with parchment. Spread the potatoes across the bottom, lay the chicken on top, and tuck lemon slices between the chicken and potatoes. No overlap — where pieces stack, they steam instead of crisp.
Step 4: Roast Hot: Preheat the oven to 200°C (400°F) and roast 22–25 minutes. The potatoes are fork-tender and the chicken is done when its center hits 74°C (165°F). For the last 3 minutes, turn on the broiler to crisp the chicken further.
Step 5: Lemon Juice Finish: Out of the oven, squeeze the juice of half a lemon over the chicken and potatoes, then rest 3 minutes so the juices redistribute. Slice and plate; extra lemon slices can be discarded or used as garnish.