In a bowl, whisk together buttermilk, lemon juice, lemon zest, garlic, honey, mustard, salt, and pepper until smooth

Add the chicken to the marinade and coat thoroughly, letting it sit until slightly infused with flavor

Toss the carrots, broccolini, and shallots with olive oil and a pinch of salt until lightly coated

Spread the vegetables evenly across the sheet pan, leaving space for the chicken

Place the marinated chicken on the pan, allowing excess marinade to drip off

Roast in the oven until the chicken is golden and cooked through and the vegetables are tender with lightly crisp edges

Check that the chicken is juicy inside and reaches safe doneness, with a lightly caramelized surface

Remove from the oven and let rest briefly so the juices settle before serving