In a mixing bowl, combine all-purpose flour, margarine, and granulated sugar until resembling coarse crumbs. Grease a 9x13-inch baking pan and press the mixture evenly into the bottom.

Preheat the oven to 350°F (175°C) and bake the prepared crust for 10 minutes until golden brown. Remove and cool completely.

While the crust cools, boil water and soften the chopped rhubarb in it for 10 minutes. Drain thoroughly to avoid sogginess.

Spread the drained rhubarb evenly over the cooled crust to provide a tart flavor layer.

In a separate bowl, whisk together egg yolks, granulated sugar, and salt. Gradually stir in milk and a touch of flour until smooth.

Pour the filling over the rhubarb layer and bake for 40 minutes until set and lightly golden.

In a clean bowl, beat egg whites with cream of tartar until soft peaks form. Gradually add sugar until glossy and stiff peaks form.

Spread the meringue over the filling and bake for an additional 10 minutes until golden brown.

Remove from the oven and allow to cool completely at room temperature before slicing into squares.