How to Make It
Preheat oven to 450°F. Squeeze juice from 2 lemons to equal ⅓ cup; add to a large bowl. Cut remaining lemon into wedges and add to bowl along with garlic, oregano, and ⅓ cup oil. Add chicken, spooning marinade over chicken; let sit for about 15 minutes.
Toss together potatoes, onion, 1 teaspoon salt, and 1 tablespoon oil in a large bowl. Transfer to a large baking sheet and roast until just beginning to brown, 10 to 15 minutes.
Toss together Broccolini and remaining 1 tablespoon oil in a large bowl. Transfer to baking sheet with potatoes and toss to combine. Using tongs, arrange chicken mixture on baking sheet in a single layer (discard marinade); season with pepper and remaining 2 teaspoons salt. Bake until chicken is cooked through and Broccolini is browned, about 30 minutes.
Roasting lemons makes them much less sour and soft enough to eat—peel and all.