Peel and thinly slice 250 grams of onions. Heat 2 tablespoons of oil in a non-stick pan over medium heat. Add the sliced onions and cook until they are soft and golden brown, stirring occasionally. This should take about 10-15 minutes.
In a bowl, whisk together 6 large eggs, 1/4 teaspoon of black pepper, and 1/2 teaspoon of salt. Add the caramelized onions to the egg mixture and mix well.
Preheat the oven to 180°C. Grease a 9-inch round baking dish or ovenproof skillet with 1 tablespoon of oil.
Pour the onion and egg mixture into the prepared dish or skillet. Spread it evenly and sprinkle 50 grams of grated cheese (such as cheddar or mozzarella) on top.
Bake the frittata in the preheated oven for 20-25 minutes, or until the eggs are set and the cheese is melted and golden brown.
Remove the frittata from the oven and let it cool for a few minutes. Cut it into wedges and serve warm with some crusty bread or a side salad.