Prepare the oven and pan: Preheat your oven to 425°F and line a large rimmed sheet pan with parchment paper to prevent sticking and make cleanup easier.

Start the potatoes: Place the halved baby potatoes and red onion wedges on the sheet pan, then drizzle them with 2 tablespoons of the olive oil and half of the salt and pepper.

First roast: Toss the vegetables with your hands or a spatula to coat them evenly, then spread them out in a single layer and roast for 20 minutes.

Season the chicken: While the potatoes roast, pat the chicken thighs dry with a paper towel and season them on both sides with the dried oregano and the remaining salt and pepper.

Add the remaining ingredients: Place the chicken thighs skin-side up in the center of the pan, then scatter the cherry tomatoes, smashed garlic cloves, and lemon slices into the gaps around the chicken.

Final roast: Drizzle the remaining 1 tablespoon of olive oil over the tomatoes and chicken, then return the pan to the oven for 25 to 30 minutes.

Check for doneness: The dish is ready when the chicken skin is golden and crispy, the meat is no longer pink near the bone, and the tomatoes have burst and released their juices.