In a large mixing bowl, combine all-purpose flour, margarine, and granulated sugar until crumbly. Press this mixture firmly into a greased 9x13-inch pan, ensuring an even layer.

Preheat your oven to 350°F (175°C). Place the crust in the oven and bake for about 10 minutes, or until it turns a light golden brown. Allow the crust to cool completely in the pan.

While the crust cools, bring a pot of water to boil. Add the chopped rhubarb and let it soak in the boiling water for approximately 10 minutes, then thoroughly drain.

Once the crust is cool, spread the softened rhubarb evenly over the surface with a spatula.

In a clean mixing bowl, whisk together the egg yolks, granulated sugar, salt, milk, and a bit of flour until smooth. Carefully pour this filling over the rhubarb layer.

Return the pan to the oven and bake for about 40 minutes, or until the filling is set and firm to the touch.

Prepare the meringue by beating the egg whites with cream of tartar until stiff peaks form. Gradually add granulated sugar, continuing to beat until glossy.

Spread the whipped meringue over the top of the filling. Create peaks and swirls with a spatula and return the pan to the oven for another 10 minutes or until golden brown.

Allow to cool completely at room temperature before slicing into squares and serving.