Heat olive oil in a non-stick, ovenproof fry pan over medium heat, add onion and garlic.
Sauté until soft - about 5 minutes.
Add spinach and stir until wilted (2-3 minutes) and any released liquid has evaporated.
Meanwhile beat the eggs in a bowl and season.
Arrange vegetables evenly across the pan and gently pour over the eggs, sprinkle over the cheese.
Cook until the eggs are firm around the edges, about 4-5 minutes. Transfer the pan to the oven and cook until the frittata is set and golden on top, 7-10 minutes. Let cool briefly.
Notes
Leftover tip: Store the remaining frittata in an airtight container in the fridge for up to 2 days. Serve cold between two or reheat if desired.