In a mixing bowl, combine all-purpose flour, margarine, and granulated sugar until crumbly. Press this mixture evenly into a greased 9x13-inch baking pan, ensuring it forms a solid layer. Bake in a preheated oven at 350°F (175°C) for about 10 minutes, or until the edges turn golden brown. Once out of the oven, let the crust cool completely while you prepare the rhubarb filling.

Chop the rhubarb into small pieces and place them in a bowl. Pour boiling water over the chopped rhubarb, allowing it to soften for about 10 minutes. After steeping, drain any excess water from the rhubarb to prevent it from becoming too soggy.

Once the crust has cooled, evenly spread the softened rhubarb over the top, making sure to cover the entire surface.

In a separate mixing bowl, whisk together egg yolks, granulated sugar, and a pinch of salt until well combined. Gradually mix in the milk and a bit of flour.

In a clean mixing bowl, beat the egg whites with cream of tartar at high speed until stiff peaks form. Gradually add sugar while continuing to beat.

Once the filling is baked and has a slight jiggle, gently spread the whipped meringue over the filling, ensuring to cover it completely.

After baking, remove the Rhubarb Fluff from the oven and allow it to cool completely in the pan on a wire rack. Once cooled, cut it into squares and serve.