Preheat your oven to 350°F (177°C) and line an 8-inch square baking pan with parchment paper.

Combine unsalted butter, granulated sugar, all-purpose flour, and kosher salt in a mixing bowl. Mix until it resembles coarse crumbs and press into the prepared pan.

Chill the crust in the refrigerator for 30 minutes, then bake for 10-12 minutes until lightly golden.

In another bowl, whisk together granulated sugar and flour, then add eggs, heavy cream, lemon zest, lemon juice, and lemon oil. Fold in the diced rhubarb.

Pour the custard filling over the cooled crust and bake for 40-45 minutes until firm.

Prepare the Swiss meringue by combining egg whites and sugar over simmering water until warm, then whip until stiff peaks form.

Spread the meringue over the cooled lemon bars and use a kitchen torch to lightly toast until golden brown.

Let the meringue set for a few minutes before cutting into squares to serve.