Preheat the oven to 375°F (190°C).
Heat olive oil in a large skillet over medium heat. Add diced onion and cook for 3-4 minutes until softened.
Add ground beef to the skillet and cook, breaking it apart with a spoon, until browned and cooked through, about 6-8 minutes.
Stir in diced red and green bell peppers and minced garlic. Cook for another 4-5 minutes until peppers are tender.
Add paprika, dried thyme, salt, and black pepper to the beef mixture. Stir well to combine and remove from heat.
In a large mixing bowl, combine thawed diced potatoes O’Brien, sour cream, and milk. Mix until potatoes are evenly coated.
Lightly grease a 9×13 inch casserole dish. Spread half of the potato mixture evenly in the bottom of the dish.
Layer the cooked ground beef and vegetable mixture evenly over the potatoes.
Sprinkle half of the shredded cheddar and mozzarella cheeses over the beef layer.
Spread the remaining potato mixture evenly on top of the cheese layer.
Top the casserole with the remaining shredded cheddar and mozzarella cheeses.
Cover the casserole dish with aluminum foil and bake in the preheated oven for 30 minutes.
Remove the foil and bake uncovered for an additional 15 minutes, or until the cheese is melted and bubbly and the casserole is heated through.
Remove from oven and let cool for 5 minutes.
Garnish with chopped fresh parsley before serving.