Photograph: Brett Stevens
Preheat the oven to 200°C, fan 180°C, gas 6. Heat the olive oil in a frying pan and cook the onion and garlic for 5 minutes until soft. Leave to cool. Line a 31cm x 21cm baking tin with baking paper.
Beat the eggs in a large bowl, add the onion, garlic, peas, feta and basil, and season with a little salt and lots of freshly ground black pepper. Stir well to combine and pour into the prepared tin.
Bake in the oven for 25-30 minutes until set and lightly golden on top. Allow to cool slightly. Remove from the baking tin and cut into 18 pieces.
For the salsa, chop up the tomatoes and mix with the basil, olive oil and balsamic glaze. Season with salt and freshly ground pepper and serve alongside the frittata.