1.Preheat oven to 200°C.
2.Boil, steam or microwave potato until tender.
3.Lightly spray 20cm ovenproof frying pan; cook onion and garlic, stirring, over medium heat, until onion softens.
4.In a large bowl, combine potato and onion mixture with asparagus, tuna, egg, egg white and parsley.
5.Reheat same pan; spray lightly with cooking oil. Spoon frittata mixture into pan; cook, uncovered, over low heat, until egg is beginning to set around the edges. Transfer pan to oven; cook frittata until top is browned lightly and egg is set, about 20 minutes.
6.Serve frittata with rocket and a squeeze of lemon. Season to taste.
You can substitute well-drained canned asparagus for the fresh, if desired.Note