Preheat oven to 200°C. Grease a 20cm x 30cm slice pan and line the base and sides with baking paper, allowing sides to overhang.
Heat the oil in a frying pan over medium heat. Add the leek and cook, stirring, for 2 mins or until tender. Add the asparagus and silverbeet. Cook, stirring, for 3 mins or until silverbeet wilts. Season. Transfer to prepared pan.
Whisk eggs and cream in a jug. Stir in cheddar. Season. Pour cream mixture over silverbeet mixture in pan. Sprinkle with peas and fetta. Bake for 20 mins or until golden and cooked through.
Meanwhile, to make the balsamic tomato salsa, combine the tomato, oil, vinegar and pine nuts in a bowl.
Spoon the balsamic tomato salsa over the frittata. Season to serve.