Line a low-sided sheet pan with foil or coat with non-stick spray, or use a 9"x13" casserole dish.
In a small bowl or glass measuring cup, combine olive oil, lemon juice, and ¼ teaspoon of salt. Whisk to combine into a marinade.
Reserve a generous Tablespoon of the marinade. In a medium bowl, toss potato chunks with the remaining marinade.
Pour the saucy potato chunks onto the prepared pan and spread evenly. Place in oven and roast for about 8 to 10 minutes.
Add the lemon slices and garlic to the potatoes, and then make room to place the chicken thighs on the pan. (It's good if some lemon slices and garlic get lodged underneath the thighs!) Note: Now is the time to add extra veggies like asparagus or zucchini coins to the pan if you want.Brush or drizzle the remaining marinade onto the chicken thighs. Sprinkle the thighs with lemon zest, if using. Season the chicken and potatoes with remaining salt. Sprinkle with onion powder, red pepper flakes (or black pepper), and rosemary.
Return to oven and roast for 20 minutes, or until the chicken reaches 165 degrees in the thickest part. (Use an instant-read thermometer to be sure.)
Serve with a salad or vegetable side.
Notes
You'll end up with softer potatoes if you start roasting them before adding the chicken thighs to the sheet pan.Throw some zucchini or broccoli on when you add the chicken to the sheet pan and you'll end up with a one-pan dinner!You can cook everything at once, but you'll need to lower the oven temperature to 400 F. Roast the whole shebang for 30 minutes. Your potatoes may be firmer, but you just might like them better that way!