Preheat the oven to 350 degrees.
Mix the soft room temperature butter, flour, and sugar until crumbly. Pat into a 9 x 13 pan and bake for 10 minutes or until the crust is light golden brown.
While the crust is baking, peel and finely chop 5 cups of rhubarb. Place the rhubarb in a bowl and cover it with the sugar.
Separate the 6 eggs and add the yolks in with the rhubarb, and sugar. Add in the salt, flour, milk, and melted butter.
Pour the rhubarb mixture over the baked crust and bake at 350 degrees for 45 minutes until the custard is set.
Beat the egg whites until they start to get frothy with a handheld mixer, or in a stand mixer. Then gradually add in the sugar, vanilla, cream of tartar, and sea salt until it becomes thick and stiff peaks form. Be careful not to overmix the meringue, you want to stop and test once in a while until you have stiff peaks that will hold their shape. Over-mixing the meringue can create a dense, dry foam-like meringue that can easily crumble and deflate.
Spread the meringue over the hot custard and bake an additional 10-15 minutes or until the meringue is light brown.
You can serve it while still warm or once it has chilled in the refrigerator. Store in the refrigerator for up to a week.