In a medium saucepan, combine the heavy cream, whole milk, and 3/4 cup granulated sugar. Heat over medium heat, stirring occasionally, until the sugar dissolves and the mixture just begins to simmer around the edges (do not boil). Remove from heat.

In a separate large bowl, whisk the egg yolks until light yellow and slightly thickened. Gradually temper the egg yolks by slowly pouring about 1/2 cup of the hot cream mixture into the yolks while whisking continuously. This prevents the eggs from scrambling.

Pour the tempered egg yolk mixture back into the saucepan with the remaining hot cream mixture. Return to medium-low heat and cook, stirring constantly with a wooden spoon or spatula, until the mixture thickens enough to coat the back of the spoon (about 5-8 minutes). Do not let it boil.

Remove the custard from heat and stir in the vanilla extract. Pour the custard through a fine-mesh sieve into a clean bowl to remove any potential cooked egg bits, ensuring a smooth base. Cover the bowl with plastic wrap, pressing it directly onto the surface of the custard to prevent a skin from forming. Chill in the refrigerator for at least 4 hours, or preferably overnight, until thoroughly cold.

For the Strawberry Puree: In a blender or food processor, combine the hulled and quartered strawberries, 1 tbsp granulated sugar, and 1/2 tbsp lemon juice. Blend until smooth. Taste and adjust sweetness if desired. Set aside.

For the Mango Puree: In a clean blender or food processor, combine the peeled and diced mango, 1 tbsp granulated sugar, and 1/2 tbsp lime juice. Blend until smooth. Taste and adjust sweetness if desired. Set aside.

Once the custard base is thoroughly chilled, pour it into your ice cream maker and churn according to the manufacturer's instructions. This usually takes about 20-30 minutes, until the ice cream is the consistency of soft-serve.

During the last 5 minutes of churning, or immediately after churning, gently swirl in spoonfuls of the strawberry puree and mango puree into the ice cream. You can alternate layers as you transfer the soft ice cream to an airtight freezer-safe container for a more defined swirl.

Cover the container tightly and freeze for at least 4-6 hours, or until firm, before serving. For best results, let the ice cream sit at room temperature for 5-10 minutes before scooping.