Stir together lemon juice, zest and sugar until sugar is dissolved. In a large bowl, whisk egg yolks and cream until blended. Stir in lemon juice mixture. Pour mixture into four 1/2 cup custard cups or ramekins.
Place a rack in the insert of a large slow cooker, or crush a sheet of aluminum foil into a ring shape and place that on the bottom of the insert. Carefully place cups on the rack (if using foil, you may need to make two rings, one slightly smaller inside the other). Pour about 1 inch of hot water into the cooker, being careful not to splash water into the cups.
Cover and cook on high for 2 hours, or until the creams are softly set and slightly jiggly in the center. Uncover and let stand for 10 minutes. Carefully remove the cups from the cooker, wearing sturdy rubber gloves so as not to burn your fingers.
Cover and refrigerate until chilled, 2 hours, or up to 3 days, before serving.