Creamy Strawberry Gratin

Ingredients

• 1 1/2 cups milk
• 4 lemons, peel removed with a vegetable peeler, no white pith remaining
• 5 egg yolks
• 1 teaspoon cornstarch
• 1/3 cup sugar
• 2 tablespoons all-purpose flour
• 1/2 teaspoon vanilla extract
• 1 tablespoon lemon juice
• 1 tablespoon unsalted butter, room temperature
• 1/2 cup mascarpone
• 3 cups strawberries, hulled and halved
• 2 tablespoons confectioner's sugar