Heat oven to 200°C or 180°C fan. Combine cornflour and 1/4 cup of milk in a jug. Place remaining milk in a saucepan and bring to boil on high heat. Whisk in cornflour mixture and bring to boil. Cook for 1 minute, stirring, until sauce thickens.
Stir in ¼ cup of cheese, corn, tuna, risoni, peas, green onion and lemon juice. Season with pepper and spoon into a 6-cup ovenproof dish or four 1 1/2-cup ramekins. Sprinkle with combined remaining cheese and breadcrumbs. Bake for 20 minutes, until golden. Serve with salad.