For chicken with tender meat and a crisp, golden brown skin in about an hour, roasting it in a high-heat oven, along with carrots, leeks, and potatoes, is the way to go for a tasty one-pot dinner.
Preheat oven to 450 degrees, with racks set in middle and lower thirds. On a large rimmed baking sheet, toss potatoes, carrots, shallots, garlic, and a few sprigs of thyme with 2 tablespoons oil. Season with salt and pepper. Set aside.
In an ovenproof skillet, toss onions with remaining tablespoon oil. Place chicken, breast side up, on top of onions. Season cavity and exterior of chicken with salt and pepper; stuff with a few sprigs of thyme. Scatter remaining thyme over chicken and onions. Using kitchen twine, tie legs together; tuck wing tips under bird.
Roast chicken on middle rack and vegetables on lower rack, occasionally brushing chicken with pan juices and tossing vegetables, until an instant-read thermometer inserted in thickest part of a thigh (avoiding bone) registers 165 degrees, 50 to 60 minutes.
Let chicken rest 10 minutes; carve, and serve with vegetables and pan juices.
Roast chicken with onions separately from the rest of vegetables so they can all have enough room to brown nicely. You could use fresh rosemary instead of thyme.