In a large mixing bowl, combine all-purpose flour, margarine, and granulated sugar. Mix until the texture resembles coarse crumbs, and then press the mixture firmly into a greased 9x13-inch baking pan.

Preheat your oven to 350°F (175°C) and bake the crust for about 10 minutes until light golden.

While the crust is cooling, prepare the chopped rhubarb by pouring boiling water over it for about 10 minutes, then drain and set aside to cool slightly.

Spread the softened rhubarb evenly over the crust, ensuring uniform distribution.

Whisk together egg yolks, sugar, and salt in a separate bowl, then gradually mix in milk and a bit of flour to make a smooth filling.

Pour the filling over the rhubarb layer and spread evenly, then bake for about 40 minutes until set with a slight jiggle.

Beat the egg whites with cream of tartar until stiff peaks form, then gradually add sugar until glossy and holds shape.

Spread the whipped meringue over the filling and bake again for about 10 minutes until golden-brown.

Allow the Rhubarb Fluff to cool completely in the pan on a wire rack, then slice into squares and serve.