In a small bowl, add the sardines in oil, the eggs and mix until combined.
Season with a pinch of salt, a pinch of black pepper, the sweet paprika and mix everything very well.
In a skillet over medium heat, add a drizzle of olive oil.
Add the red onion cut into slices and sauté for 2 minutes.
Then, add the egg mixture we prepared and spread it out.
Cover the skillet and let it all cook for 5 minutes.
After this time has passed, with the help of a plate, turn the omelet over.
Add the grated mozzarella cheese and the grated cheddar cheese.
Cover the skillet again and let everything fry until the cheese melts completely.
Serve.