Preheat oven to 400°F (200°C).
In a large bowl, combine the chicken thighs, lemon juice, dijon mustard, oregano, paprika, salt, black pepper, garlic powder, lemon zest, smashed garlic, and 3 tbsp olive oil. Toss to coat the chicken.
Spread the chicken and potatoes out on a large baking sheet. Sprinkle the parmesan cheese over the chicken and potatoes.
Bake for 35-40 minutes, until the chicken is cooked through and the potatoes are tender.
While the chicken and potatoes are baking, make the garlic parmesan butter. In a small bowl, combine the butter, 1 cup parmesan cheese, 5 garlic cloves, and 2 tsp black pepper. Mix well.
Also make the roasted garlic tzaziki. Roast the 2 garlic heads with 2 tbsp olive oil and 2 tsp salt at 400°F (200°C) for 30-40 minutes, until very soft. Allow to cool, then squeeze the roasted garlic out of the skins. In a bowl, combine the roasted garlic, dill, grated cucumber, greek yogurt, 1 tsp salt, lemon juice, and 1 tbsp olive oil. Mix well.
When the chicken and potatoes are done, drizzle the garlic parmesan butter over the top. Garnish with fresh parsley and dill.
Serve the one-pan chicken and potatoes with the roasted garlic tzaziki on the side.
