1. Prepare an ice-water bath. Whisk lemon juice, sugar, flour, and salt in a small saucepan over medium-high heat. Bring to a boil; whisk constantly for 1 minute, until it thickens. Transfer to a heatproof bowl set in ice-water bath to cool completely, stirring occasionally. Meanwhile, whisk cream in a mixer on medium speed until stiff peaks form, about 3 minutes. Gently fold whipped cream into juice mixture in thirds. Add lemon zest and gently fold to combine. Refrigerate lemon cream, up to overnight.