Preheat oven to 500. Yes, 500. We're roasting here. (475 with convection would also work.)
Cut potatoes in half. (With tiny potatoes, use more and leave whole.) Add to cold salted water and bring to a simmer, then cook about 5 minutes to parboil them.
Stem and chop the parsley. Add it to a bowl and juice 2 lemons into it. Press or grate the garlic into the bowl. Add 1–2 Tbsp olive oil, salt, and pepper, and mix together.
Reserve half of the mixture for later. Put the other half into a large bowl. Add garlic powder and oregano. Add more olive oil. Mix.
Drain the potatoes but do not rinse. Toss thoroughly with chicken thighs in the marinade bowl to coat everything with the slurry.
Spread out on baking tray with parchment paper. Slice the last lemon and arrange the slices on top.
Roast for 30 minutes, then flip/rotate as needed, then roast for about another 30 minutes. Use tongs to squeeze the roasted lemons.
Transfer to a clean bowl (not the one that had raw chicken, unless you wash it) and toss carefully with the reserved half of the marinade. Don't mash the potatoes!
Plate with arugula underneath so the marinade and heat can wilt it a bit. Drizzle with olive oil.
