Preheat the oven to 375 degrees. In a large bowl, combine the ground beef, rolled oats, egg, milk, onion, garlic, Italian seasoning, salt, and pepper. Mix gently until just combined.
Press the mixture into a loaf pan and bake for 50 to 55 minutes until the internal temperature reaches 160 degrees.
While the meatloaf bakes, bring the water to a boil in a medium saucepan. Stir in the polenta and salt, then reduce heat to low. Cook, stirring often, for 20 to 25 minutes until thick and creamy. Stir in the butter and Parmesan cheese at the end.
Warm the marinara sauce in a small saucepan over low heat.
Let the meatloaf rest for 10 minutes before slicing. Serve slices over a scoop of polenta and spoon marinara on top.