Add the raspberries, sugar and 1 teaspoon of water to a small saucepan and heat over medium-low. Use the back of a wooden spoon to slightly break up the raspberries. Cook, stirring regularly, until the raspberries have completely broken up and are saucy, 3 to 4 minutes. Strain mixture over a bowl slightly pressing the pulp to release as much liquid as possible. Discard the pulp and seeds. Cool liquid completely.
Add the sweetened condensed milk, lemon zest and lemon juice to a large bowl and stir to combine. In a separate large bowl add the heavy cream and whip with a hand mixer until stiff peaks form, about 5 minutes. In batches, delicately fold the whipped cream into the sweetened condensed milk mixture, making sure it's fully incorporated before adding another batch.
Pour half of the whipped cream mixture into a 9-by-5 loaf pan and spoon 1 1/2 tablespoons of the raspberry sauce over the top. Using a wooden skewer or a chop stick, swirl and slightly stir the raspberry sauce into the whipped cream mixture. Top with the remaining whipped cream mixture and spoon another tablespoon of raspberry sauce on top. Once again, swirl the sauce into the whipped cream mixture. Loosely cover the loaf pan with plastic wrap and place in the freezer. Freeze until solid, 6 to 8 hours, preferably overnight. Scoop into bowls and serve.