Cut off tops of lemons; dig out pulp with small spoon. Remove pips and reserve pulp and juice.
Mash sardines or tuna fish to a smooth paste with butter and mustard, and season to taste with paprika and freshly ground black pepper. Stir in juice and pulp of lemons together with stiffly beaten egg white. Correct seasoning and stuff lemons with this mixture. Chill. Top with a sprig of fresh thyme, a bay leaf or a small green leaf, and serve in egg cups.