Heat the oven to 425°.
Zest the lemon and reserve for later. Juice the lemon so you have 1/4 cup juice (I cut the lemon in half, dig a fork into the flesh and squeeze and rotate the lemon over a sieve to catch the seeds).
Whisk the juice with the oil, salt, garlic powder, pepper and 1 tablespoon minced rosemary. Set aside 3 tablespoons separately.
Scrub the potatoes under running water and dry. Use a small knife to nick out any nibbly bits on the skin. If large, cut the potatoes lengthwise into quarters. If small, cut in half lengthwise. Slice the potatoes into 1/2" pieces.
Peel and thinly slice the shallot into rings.
In a large bowl, toss the potatoes, shallot and frozen peas with the larger amount of dressing (not the 3 tablespoons). It's okay if the shallot rings separate.
Spread the veggies out on a baking sheet, making sure all the potatoes are touching the sheet.
Pat the chicken thighs dry and cut away any excess skin if necessary. Place on top of the veggies. Re-whisk the remaining 3 tablespoons sauce if necessary (if the ingredients don't look blended anymore) and drizzle over the chicken.
Bake for 30-45 minutes, depending on the size of the chicken thighs, until they reach 165°. If you'd like the chicken skin a little browner/crispier, broil for 1-5 minutes (depending on your broiler; keep an eye on them).
While the chicken and veggies are baking, finely mince the reserved lemon zest with the remaining 2 tablespoons minced rosemary. Once the sheet pan is out of the oven, sprinkle over the zest/herb mixture and serve, spooning the pan juices over the chicken and veggies.