Preheat the oven to 350 degrees F. Grease an 8x8" baking pan and set aside.
Cook the pasta according to package instructions, but only for ¾ of the recommended time (i.e. for 8 minutes, cook for 6). Drain and reserve for later.
While the pasta is cooking, heat 1 teaspoon olive oil in a large skillet over medium heat. Add the cubed chicken and cook for 7-9 minutes, flipping halfway through, or until the chicken is cooked through. Be sure to season the chicken with salt and pepper during the cooking process. Transfer to a clean plate.
In the same skillet, heat the rest of the olive oil and add the sliced leeks and garlic. Cook for a few minutes, or until tender. Add the chicken, milk, dijon, lemon juice, thyme, and ½ cup of the parmesan cheese. Season with salt and pepper to taste. Stir and cook for a few more minutes.
Add the cooked pasta to the greased pan. Pour the leek and chicken mixture on top, making sure to pour in the liquid from the skillet, and stir. Sprinkle the rest of the parmesan cheese on top and pop in the oven for 15-20 minutes. Enjoy warm!
Notes
To make this in advance, follow steps up until putting it in the oven. Cover with foil or plastic wrap and store in the fridge until you want to cook it (up to 24 hours).
Leftovers can last in an airtight container in the fridge for 3-5 days. Reheat in a skillet, with about a tablespoon of olive oil, for about 5 minutes until warmed through.