Preheat your oven to 375°F (190°C).
In a large skillet over medium heat, add the olive oil and sauté the chopped onion and minced garlic until translucent (about 3-4 minutes).
Add the diced potatoes to the skillet and stir to coat. Cook for about 10 minutes, stirring occasionally, until the potatoes start to soften.
In the same skillet, push the potatoes to one side and add the sausage. Cook until browned and cooked through, breaking it up into smaller pieces (about 5-7 minutes).
Stir the sausage into the potatoes, then sprinkle with paprika, oregano, salt, and pepper. Mix well.
In a separate bowl, combine the milk and half of the cheddar cheese. Pour this mixture over the potato and sausage mixture in the skillet and stir until combined.
Transfer the mixture to a baking dish and spread it evenly. Top with the remaining cheddar cheese and mozzarella cheese.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden.
Let it cool for a few minutes before serving. Garnish with chopped parsley if desired.