Make the biscuits: Line a heavy baking sheet with parchment. Sift the flour, sugar, baking powder, and salt into a large bowl. Add the lemon zest and poppyseeds and toss with a fork to combine. Cut the butter into the flour mixture with a pastry cutter or a fork until the largest pieces of butter are the size of peas. Combine the cream and lemon juice in a liquid measure. Make a well in the center of the flour mixture and pour the cream mixture into the well. Mix with a fork until the dough is evenly moistened and just combined; it should look shaggy and still feel a little dry. Gently knead by hand five or six times to pick up any dry ingredients remaining in the bottom of the bowl and to create a loose ball.

Turn the dough out onto a lightly floured work surface and pat it into an 8-inch square, 3/4 to 1 inch thick. Transfer the dough to the parchment-lined baking sheet, cover with plastic wrap, and chill for 20 minutes. Meanwhile, heat the oven to 425 degrees F. Remove the dough from the refrigerator and trim about 1/4 inch from each side to create a neat, sharp edge (a bench knife or a pastry scraper works well, or use a large chef's knife, being sure to cut straight down). Cut the dough into 9 even squares (about 2-1/2 inches square) and spread them about 2 inches apart on the baking sheet. With a pastry brush or the back of a spoon, brush each biscuit with a thin layer of cream and sprinkle generously with sugar. Bake until the biscuits are medium golden brown on top and the bottoms are golden brown, 18 to 20 minutes.

Meanwhile, prepare the berries: Toss the berries with 1 tablespoon sugar and taste. If they're still tart, sprinkle with another 1 to 2 tablespoons sugar. Let sit at room temperature until the sugar dissolves and the berries begin to release their juices, at least 30 minutes but no more than 2 hours.

Whip the cream: Pour the cream into a cold mixing bowl and beat with a hand mixer until it begins to thicken. Add the sugar and lemon juice and, using a whisk, continue to beat by hand until the cream is softly whipped or until the whisk leaves distinct marks in the cream; it should be soft and billowy but still hold its shape.

Assemble the shortcakes: While the biscuits are still warm, split them in half horizontally with a serrated knife. For each serving, set the bottom half of a biscuit on a plate. Scoop about 1/2 cup of the berries and their juices over the biscuit. Add a generous dollop of whipped cream and cover with the top half of the biscuit. Top with a small dollop of cream and garnish with lemon zest.