Cut 5 lemons lengthwise into halves, then use a spoon to lift their pulp into a fine-mesh sieve positioned over a bowl, loosening the edges with a knife if needed. Keep the outer shells intact so they can hold the cream without leaking.

Press the pulp against the sieve, measure ⅓ cup of the collected juice, and reserve any remaining juice for another use. The measured juice should be free of seeds and coarse pulp.

Arrange the empty shells with their openings facing upward on a tray or in a muffin tin, then zest the remaining lemon. Each shell should sit securely without rocking when the tray moves.

Add the cream, sugar, 1 tablespoon lemon zest, and salt to a medium saucepan, then heat over medium heat while stirring until the sugar dissolves and a gentle boil begins. Look for bubbling foam across the surface rather than an occasional bubble at the edge.

Maintain a gentle simmer for the full 3 minutes without stirring. The cream should concentrate slightly, with a light coating on a spoon and possibly a subtle shift toward pale ivory.

Take the saucepan off the burner and strain its contents into a heatproof measuring cup or pitcher through a fine-mesh sieve. The sieve should catch the zest while the fragrant cream flows into the pitcher.

Whisk the measured lemon juice into the strained cream until evenly incorporated. The mixture should gain a little body, though it will still pour readily.

Fill the prepared lemon shells while the mixture remains warm and fluid, using very gentle low heat only if it begins setting before you finish pouring. It should flow smoothly without bubbling, leaving a level surface in each cup.

Transfer the tray to the refrigerator and leave the possets uncovered for at least 4 hours, continuing to chill if they have not fully set. The finished cream should hold a spoonful rather than flow like a sauce.