Begin by preheating your oven to 375°F (190°C). While the oven heats up, grab your chicken thighs. Trim any excess fat and cut them into bite-sized pieces. In a skillet, heat 1 tablespoon of olive oil over medium-high heat. Once the oil is shimmering, add the chicken pieces. Season with salt, black pepper, smoked paprika, and garlic powder. Sauté until golden brown and cooked through, about 5-7 minutes.

In the same skillet, add the remaining olive oil and toss in the broccoli florets and sliced leeks. Cook these until the broccoli is bright green and slightly tender, around 3-4 minutes. You want the veggies to maintain some crunch as they will continue to cook in the oven.

In a mixing bowl, combine the heavy cream, yogurt, mustard, and half of the Parmesan cheese. Whisk until well mixed and smooth. This creamy mixture will provide a luscious texture to our casserole.

In a large mixing bowl, combine the sautéed chicken and vegetables. Pour the creamy sauce over the mixture and stir until everything is well coated. Transfer this mixture into a greased baking dish.

Sprinkle the remaining Parmesan cheese over the top, followed by the cheddar cheese. This will create a delicious cheesy crust as it bakes.

Cover the dish with aluminum foil and bake in the preheated oven for 20 minutes. After this time, remove the foil and bake for an additional 10-15 minutes until the cheese is bubbly and golden.

Once done, take the casserole out of the oven and let it cool for a few minutes before serving. This dish can be enjoyed hot, and it will smell divine!