Preheat the oven to 425 degrees F.

In a large bowl, whisk together olive oil, lemon zest, lemon juice, minced garlic, Dijon mustard, dried oregano, sweet paprika, kosher salt, and freshly ground black pepper until well combined.

Add the halved fingerling potatoes, lemon slices, and large chunks of yellow onion to the bowl or a large sheet pan. Pour about 1/3 cup of the marinade over these and toss to coat evenly. Spread them out on the sheet pan with the flesh side of the potatoes down. Roast in the oven for 25 minutes.

While the potatoes roast, add the chicken thighs to the remaining marinade and coat well. Set aside.

After 25 minutes, remove the sheet pan from the oven and flip the potatoes. Allow any excess marinade to drip off the chicken. Nestle the marinated chicken thighs onto the sheet pan between the potatoes.

Return the baking sheet to the oven and roast for an additional 15 to 20 minutes until the chicken is cooked through and golden brown, and the potatoes are tender.

Remove from oven and garnish with crumbled feta cheese, chopped parsley, chopped dill, red pepper flakes if desired, and season with extra salt and pepper. Serve warm.