Preheat your oven to 350°F (175°C). In a mixing bowl, combine all-purpose flour, margarine, and granulated sugar. Blend until the mixture resembles coarse crumbs. Press this dough into a greased 9x13-inch pan.

Bake for about 10 minutes until it turns a light golden brown. Allow it to cool completely.

In a medium saucepan, bring water to a boil and add chopped rhubarb. Soften for 10 minutes, then drain and cool slightly before layering on the crust.

Spread the cooled rhubarb evenly over the crust.

In a large mixing bowl, whisk egg yolks, granulated sugar, and salt. Slowly add milk and mix until smooth. Incorporate a bit of flour to thicken.

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

Beat egg whites and cream of tartar until stiff peaks form. Slowly add sugar and continue beating until glossy.

Spread meringue over the filling and bake for another 10 minutes until lightly golden.

Allow to cool completely on a wire rack before slicing into squares.