Preheat your oven to 400°F (200°C). Grease a 9x13 inch casserole dish with butter or cooking spray.

Place the cubed potatoes in a large pot and cover with cold, salted water. Bring to a boil and cook for 5-7 minutes, until they are just slightly tender (par-cooked). Drain well and set aside.

While potatoes are cooking, heat olive oil in a large skillet over medium-high heat. Season the chicken cubes with 1/2 tsp salt and 1/4 tsp pepper. Add the chicken to the skillet and cook for 5-6 minutes, until browned on all sides. Do not cook through. Remove the chicken from the skillet and set aside.

In the same skillet, reduce heat to medium. Add the chopped onion and cook for 4-5 minutes until softened. Add the minced garlic and cook for another minute until fragrant.

Melt the butter in the skillet with the onions and garlic. Whisk in the flour and cook for 1 minute, stirring constantly, to form a roux.

Gradually pour in the warm milk and chicken broth, whisking continuously to prevent lumps. Bring the sauce to a simmer and cook for 3-4 minutes, until it has thickened.

Remove the skillet from the heat. Stir in the Dijon mustard, smoked paprika, dried thyme, the remaining 1/2 tsp salt, and 1/4 tsp pepper. Add the 1 cup of cheddar and 1/2 cup of Gruyère cheese, stirring until the cheese is completely melted and the sauce is smooth.

In a large bowl, combine the par-cooked potatoes and the browned chicken. Pour the cheese sauce over the top and gently stir until everything is evenly coated.

Transfer the mixture to your prepared casserole dish and spread it into an even layer.

Sprinkle the remaining 1/2 cup of cheddar cheese and the panko breadcrumbs evenly over the top.

Bake for 25-30 minutes, or until the sauce is bubbly and the top is golden brown. If the top browns too quickly, you can loosely cover it with foil.

Remove from the oven and let the casserole rest for 10 minutes before serving. This allows the sauce to set. Garnish with fresh parsley and serve warm.