Preheat your oven to 350°F (177°C). In a mixing bowl, combine unsalted butter, granulated sugar, all-purpose flour, and kosher salt. Mix until the mixture resembles coarse crumbs. Press this crumbly blend evenly into the bottom of an 8-inch square pan, and chill for 30 minutes. Bake the crust for 10-12 minutes, then remove from the oven and cool.

In a separate bowl, whisk together flour and sugar until well combined. Add large eggs, heavy cream, lemon zest, fresh lemon juice, and lemon oil, mixing until smooth. Gently fold in the diced rhubarb. Pour this custard over the cooled shortbread crust and bake for 40-45 minutes, or until the custard is set. Allow it to cool before refrigerating for at least 1 hour.

Once the custard layer is chilled, start your Swiss meringue. In a saucepan, combine egg whites, granulated sugar, and vanilla extract. Heat over low until the sugar dissolves, stirring gently. Transfer to a stand mixer and beat on high until stiff peaks form, about 5-7 minutes.

Spread or pipe the Swiss meringue over the cooled lemon rhubarb custard, then toast it using a kitchen torch or under a broiler on low for 1-3 minutes, until golden brown.

Allow the Lemon Rhubarb Bars to cool before cutting them into squares. Serve chilled with powdered sugar or whipped cream. Store leftovers in an airtight container in the refrigerator for up to 5 days.