Preheat oven to 425° and spray a sheet pan with olive oil.
In a small bowl, combine olive oil, garlic, lemon zest, salt and pepper. Whisk to combine. Pour 3/4 of it into a large ziplock bag and add your chicken thighs. Close bag, give it a shake and let marinate while you prep veggies. You can marinate it longer of course by prepping ahead of time but you don't have to.
Dice the potatoes, cut carrots into strips and peel and quarter the onion. Put in a bowl and toss with remaining olive oil mixture.
Take the chicken out of the bag, letting the excess olive oil drip off before placing on sheet pan. (I like to use a slotted spoon or even a fork to get any of the garlic and herbs that are left in the bag to place on the chicken.) Using a slotted spoon, place the veggies around the chicken, dispersing evenly. Try not to let the veggies stack on top of each other too much because we want them to get a good crisp. Top each piece of chicken with a tbsp of butter
Bake for 45-55 minutes until internal temp of chicken reaches 165° and is golden brown. I shake the pan about halfway through to move the veggies around. Turn the broiler on for 3-4 minutes at the end to get the tops nice and brown and crispy.