Cook potatoes with 45 ml (3 tbsp) water, covered on HIGH power for 6-8 mins. or until cooked. Drain.
Place butter and onion in a bowl, cover and cook on HIGH power for 4 mins. or until soft.
Add potatoes and mash.
Stir in tuna, corn, hard-boiled egg, parsley and lemon juice, season well.
Shape into 8 cakes and coat in beaten egg and Breadcrumbs.
Place on grill rack and cook on Combination: GRILL 1 and SIMMER power for 16-18 mins. or until brown, turning halfway.