Hard boil the eggs for 10 minutes. Peel and slice the onion and fry gently in the knob of butter in a small saucepan over a moderate heat, for 4-5 minutes, until soft and cooked. Grate the cheese.
For the cheese sauce: either mix the flour or cornflour into a smooth paste with a little of the milk in a small basin. Boil the rest of the milk and pour onto the flour mixture, stirring all the time. Then pour the whole mixture back into the saucepan and stir over the hat until the mixture thickens. Stir in the butter and beat well. Add the 1oz (25g) grated cheese, salt, pepper, and mustard, or make up the packet sauce mix.
Put the onion into a greased ovenproof dish. Slice the cold, peeled hard-boiled eggs, and arrange on top of the onion. Cover with the cheese sauce and sprinkle with the rest of the grated cheese. Brown under a hot grill for a few minutes, until the cheese is melted, crisp, and bubbly.