Step 1

Prepare lemon curd filling: Cut butter into small pieces and set aside.

Step 2

Separate eggs, placing yolks in a heat-proof bowl and whites in glass, metal, or ceramic bowl. Cover egg whites and leave on the counter to come to room temperature for the meringue.

Step 3

Add lemon zest, salt, sugar, and lemon juice to egg yolks. Whisk until thoroughly combined.

Step 4

Add 1 to 2 inches water to a saucepan over low heat; bring to a simmer. Place bowl of filling over the simmering water. Cook, stirring constantly with a spatula, until mixture is very hot to the touch and thick enough to coat the back of a spoon, 10 to 15 minutes.

Step 5

Stir in butter, 3 or 4 pieces at a time, waiting until butter has melted before each addition. Remove from the heat and pour filling into a bowl to cool, about 15 minutes. Lay plastic wrap on top of the filling, pressing down so it doesn't develop a skin, and place in the refrigerator until well chilled, 2 to 3 hours.

Step 6

Preheat the oven to 250 degrees F (120 degrees C). Line a baking sheet with a silicone liner, such as Silpat®. Lightly spray with cooking spray.

Step 7

Begin meringue: Mix cornstarch into sugar and set aside.

Step 8

Add cream of tartar, lemon juice, and vanilla to the reserved egg whites. Whisk with a large whisk until very soft peaks form, about 3 minutes. Whisk in sugar mixture, 2 tablespoons at a time, whisking about 1 minute between each addition, until incorporated. When all sugar mixture has been added, continue to whisk until relatively stiff peaks form and whites are firm enough to hold their shape.

Step 9

Use 2 spoons or a piping bag to transfer whites onto the prepared baking sheet, forming a ring of dollops about 8 or 9 inches across. Fill in the middle with meringue, then go back around the edges adding more meringue to build up the outside edge.

Step 10

Bake in the center of the preheated oven for exactly 1 hour. At 1 hour, turn off the oven and crack the door. Let meringue sit in the oven until its cooled completely, about 1 hour.

Step 11

Meanwhile, prepare blueberry sauce: combine 1 cup blueberries, sugar, and water in a saucepan over medium heat; bring to a simmer. Cook, stirring occasionally, until blueberries collapse and liquid thickens, about 10 minutes.

Step 12

Remove from the heat and pass through a fine mesh strainer. Place in the refrigerator until fully chilled, about 45 minutes.

Step 13

Very carefully transfer cooled meringue to a serving platter using 2 large spatulas if necessary; you may get some cracks but that's okay. Use a spoon to crack the center a bit to make room for the filling.

Step 14

Fill the center with chilled lemon curd filling. Place remaining blueberries over top and drizzle with blueberry sauce. Garnish with mint.