Add watermelon, sugar, and lemon juice to a food processor. Puree until smooth. Transfer to a freezer-safe shallow dish such as a 9x13 glass or metal baking dish and freeze. After 30 minutes, scrape the mixture with a fork, moving ice crystals from the edges to the center and breaking up large pieces. Repeat scraping every 30 minutes until fully frozen, approximately 3-4 hours. Before serving, sprinkle granita with flaked sea salt.

In a heavy saucepan over medium heat, combine the milk, sugar, and 1 cup of heavy cream. Heat until just beginning to bubble around the edges. Remove from heat and stir in mint leaves. Cover and steep for 1-2 hours.

In a medium bowl, whisk together the egg yolks until smooth. Reheat the mint-infused mixture and slowly pour 1/3 of it in a thin stream into the yolks while whisking.

Return the tempered egg yolk mixture to the saucepan. Heat gently while stirring continuously until thickened and the mixture coats the back of a spoon. Strain through a fine mesh sieve into a bowl and mix in the remaining 1 cup of heavy cream. Chill completely.

Follow the instructions on your ice cream maker to churn the chilled mixture until thick and creamy.

In a serving dish, alternate layers of salted watermelon granita and scoops of fresh mint ice cream. Serve immediately.