tiny squeeze lemon juice
1 heaped tsp diced red onion or shallot
1 spring onion, thinly sliced
1 tbsp mayonnaise
80g tinned tuna, drained
salt and freshly ground black pepper
1 tsp olive oil
2 thick slices white or wholemeal bread
1 tbsp butter, softened
50g/1戮oz extra mature or mature cheddar, grated