In a medium oven-proof skillet, heat olive oil over medium heat. Add the diced onion, and saute until it's translucent. Add minced garlic, and cook for 1 minute or until it's fragrant.
Add diced tomatoes with the juices, oregano, basil, salt, and pepper. Cook on medium heat, and crush the tomatoes with a wooden spoon as you cook them. Simmer for 10-15 for the juices to reduce.
Meanwhile, cook the gnocchi. Bring a pot or saucepan with water to a boil, salt the water, and drop the gnocchi. Cook for a few minutes until the gnocchi float to the surface. When they float, it means that the gnocchi is done.
Using a slotted spoon or a spider, remove the gnocchi from the cooking water to drain.
Add the cooked gnocchi to the cooked sauce and mix so that they're well coated.
Top the gnocchi with shredded mozzarella cheese, and sprinkle with grated parmesan.
Broil for 5-8 minutes or until the mozzarella bubbles and crisps up a little. The gnocchi at the top will also become crispy. Garnish with fresh basil leaves and serve immediately.