Boil– In a small saucepan, heat the cream and sugar, stirring to dissolve the sugar. Bring to a boil, then adjust the heat so it doesn’t boil over and keep it at a medium boil for 5 minutes.: As the cream warms, you’ll notice a sweet dairy scent rising and tiny bubbles forming along the edges, which signals that the sugar is dissolving and the mixture is concentrating; this slow, steady simmer allows the cream to slightly thicken and develop depth without scorching, and it also helps evaporate a bit of water so the posset can set; a good sensory cue is a steady ring of small bubbles plus a faint caramel undertone at the rim, and you should hear a gentle rolling simmer rather than violent popping; the reason this matters is that the controlled boil changes the cream’s structure so it becomes naturally thick when acid is added later; a common mistake is letting the pot roar which can cause boil over and uneven cooking, so reduce heat as soon as the boil starts; if the mixture develops a burnt smell, discard and start over to avoid an off flavor.
Add lemon– Remove the pan from the heat and stir in the lemon juice, then set aside to cool for 15 minutes. Strain the mixture through a fine mesh sieve set over a large measuring cup with a pouring lip.: The second you add the bright lemon juice you will notice the aroma lift and the mixture begin to thicken slightly, the acid interacts with the warmed cream creating the silky body that defines posset; letting it cool for fifteen minutes tempers the heat so you can strain without steam buildup, and straining through a fine mesh removes any coagulated bits and seeds for a perfectly smooth texture; pay attention to the way the mixture pours, it should be viscous yet pourable, and the color will be a pale creamy lemon; a typical error is skipping the sieve which can leave grainy or lumpy bits in the finished dessert, so always strain carefully and scrape the pot to capture all the flavor.
Fill– Divide the mixture between 6 ramekins or between 10-12 lemon halves if you prefer to serve it that way. Fill your serving cups about 3/4” full so it sets up properly.: As you fill the dishes, notice the way the liquid settles into a glossy surface that will dull slightly as it cools; filling to a consistent level ensures even setting and an attractive presentation; if using lemon halves, the bright shells add charm and a hint of extra citrus aroma, but watch for minor drips while pouring to keep the edges clean; the importance here is portion control, because overfilling will prevent a proper set and underfilling can yield an overly firm center; a common slip is rushing and spilling, so pour slowly and use a measuring cup with a lip for control.
Refrigerate for at least 2 hours or until fully set, then top with whipped cream if desired, along with a lemon slice or berries for garnish.: The refrigerator’s coolness gradually firms the custard, and during this time the texture tightens into a dense, spoonable set with a delicate sheen; you’ll recognize readiness when the surface no longer jiggles and a spoon leaves a clean trail; chilling also allows the flavors to meld so the lemon sharpness becomes integrated with the rich cream; avoid placing the possets near strong smelling foods in the fridge, cover them loosely if possible to prevent odor absorption; a typical error is checking too early, which can lead to a runny center, so be patient and give it the full two hours or more if your fridge is very cold.