Preheat your oven to 180°C (350°F) and position a rack in the middle. Grease a 23cm (9-inch) ovenproof non-stick skillet or a similar-sized baking dish with butter or spray. The pan should feel slick but not greasy to the touch.

Sauté the aromatics: Heat the olive oil in the skillet over medium heat. Add the diced onion and cook for 4-5 minutes, stirring occasionally, until the pieces turn translucent and the edges just begin to brown. The kitchen should smell sweet and savoury. Stir in the minced garlic and cook for 30 seconds — you'll know it's ready when the garlic becomes fragrant but hasn't coloured.

Whisk the egg whites: In a large bowl, pour in the liquid egg whites. Add the dried oregano, salt, and pepper. Whisk vigorously for about 30 seconds until the mixture is uniformly pale yellow and slightly frothy on top. You should see small bubbles forming across the surface — that's air being incorporated, which helps the frittata rise.

Assemble in the pan: Spread the cooked onion and garlic evenly across the bottom of the skillet. Arrange the halved cherry tomatoes cut-side up, nestling them into the onion layer. Scatter the torn mozzarella pieces and half the Parmesan evenly over the top. Slowly pour the whisked egg whites over everything, tilting the pan gently so the liquid settles into all the gaps. The tomatoes and cheese should be partially submerged but still visible.

Bake the frittata: Place the skillet in the preheated oven and bake for 20-25 minutes. After 15 minutes, check the frittata — the edges should be set and pulling away slightly from the pan, while the centre still jiggles when you gently shake the skillet. Sprinkle the remaining Parmesan over the top and continue baking. The frittata is done when the centre is just set and a knife inserted into the middle comes out clean, with no wet egg clinging to it. The top should be puffed and golden in patches, and you'll hear a faint sizzle as the cheese bubbles.

Rest and garnish: Remove the skillet from the oven and place it on a wire rack. Let the frittata rest for 5 full minutes — this is crucial for the texture (see tips below). The surface will settle slightly as it cools. Scatter the fresh basil over the top and slice into 6 wedges. The slices should hold their shape cleanly without crumbling.