Preheat the oven to 220°C. Place the beans in a saucepan and cover with boiling water, then bring back to the boil and cook for 3 minutes. Drain and set aside.
Mix the cornflour with the water from the tuna in the same saucepan. Stir with a whisk until smooth, then stir in the milk. Bring to the boil, whisking vigorously until the sauce is thickened and smooth. Remove the pan from the heat.
Use a spoon to stir the corn into the sauce, then stir in the spring onions, beans and tuna. Season to taste. Divide this mixture among four individual ovenproof soufflé or gratin dishes, each about 300 ml capacity. Alternatively, turn the mixture into a 1.5 litre ovenproof dish.
Mix the tomatoes, fetta cheese and olive oil together in a bowl. Stir in the diced bread until thoroughly combined. Pile this mixture on top of the tuna sauce, pressing it on gently with your fingers so that it will stay in place without squashing the topping.
Bake the tuna and corn pots for 15 minutes or until the bread is browned and crisp, the cheese has softened and the tomatoes are lightly cooked. Underneath, the tuna mixture should be bubbling and piping hot. Serve at once.
4 portions vegetables
For an extra cheesy taste, grate some cheddar or parmesan cheese over the top of each pot before cooking them.