1) Cube Yukon potatoes and add to simmering salted water. Simmer for about 15 minutes until they are very tender. Then transfer to a bowl with 1/3 cup Land O Lakes® Butter with Canola Oil and cream. Beat with a hand mixer or stand mixer (paddle attachment) until smooth and creamy. Season with salt and pepper to taste.
2) Preheat oven to 400 degrees F. In a large skillet over medium heat, add 2 tablespoons of Land O Lakes® Butter with Canola Oil along with shallots, carrots, and celery. Cook for 6-7 minutes until vegetables soften and start to take on some color. Season with a pinch of salt and pepper.
3) Stir flour into the vegetables and cook for a minute or two to make a roux. Then slowly stir in chicken stock to make a light gravy. Add shredded chicken, peas, and thyme, and remove from heat.
4) Transfer chicken mixture to a 2 ½ quart baking dish. Cover with smashed potatoes. I find it easiest to pipe them onto the top to get an even layer but you can just spread them on with a spoon as well.
5) Top casserole with cheese (optional) and bake for 20-25 minutes at 400 degrees F. until potatoes are browned slightly and the filling is bubbling.
6) Let the parmentier cool slightly and serve while warm garnished with parsley.