Prep the ingredients by peeling and dicing the potatoes, slicing the kielbasa, and dicing the onion and mincing the garlic.
In a large skillet, heat olive oil over medium heat. Add diced onion and sauté for 3-4 minutes until soft. Add minced garlic and cook for 1 minute. Add sliced kielbasa and cook for 5-7 minutes until browned.
Prepare the potatoes by peeling and dicing them. Optionally, parboil the potatoes for 5 minutes in boiling water, then drain and set aside.
In a large mixing bowl, combine diced potatoes, cooked kielbasa mixture, heavy cream, paprika, dried thyme, salt, and pepper. Stir until well combined.
Pour the mixture into a prepared baking dish, top with shredded cheddar and mozzarella cheese. Cover with foil and bake at 375°F for 30 minutes. Remove foil and bake for an additional 15-20 minutes until cheese is bubbly and golden.
Let the casserole cool for 5 minutes before serving. Garnish with chopped fresh parsley.