In a medium-sized saucepan, combine the diced rhubarb, sliced strawberries, sugar, salt, flour, butter, and lemon juice or lemon oil. Let the mixture sit for about 15 minutes; the fruit will release its juice, and it'll be easier to cook.
Separate the eggs, reserving the whites. Cook the fruit mixture over low heat for several minutes, until the butter is melted and the mixture thins out. Stir in the cream, if you're using it.
In a small bowl, whisk the egg yolks until light. Add a bit of the hot fruit to the yolks, stir well, then add the yolk mixture to the cooking fruit. Cook over medium-low heat, stirring constantly, until the mixture comes to a boil. Remove the pan from th
Place the pie crust dough on a well-floured surface, and roll it into a 12-inch circle. Place it in a 9-inch pie pan, preferably one with sides at least 1 1/2 inches high. Crimp the edges. Prick the bottom with the tines of a fork. Place a second pie pan,
Pour the filling into the partially baked crust. Reduce the oven heat to 350°F and bake the pie for 20 minutes.
While the pie is baking, place the reserved egg whites and cream of tartar in a medium-sized bowl, and beat them till they're foamy. Add the sugar a bit at a time, beating until the meringue is shiny and holds a peak. Place the water and cornstarch in a t
Remove the pie from the oven and spoon the meringue over the filling while the filling is still hot, making sure that the meringue makes contact with the crust all around the edges. Return the pie to the oven and bake for an additional 16 to 18 minutes, o