Combine the salt, sugar, and flour in a bowl, or in a food processor. Cut the butter into the flour until it resembles very course corn meal. Add the cold water at once and work the mixture as little as possible, just until the water is absorbed enough to make a ball of dough.
Squash into a hockey puck shape, wrap in waxed paper and chill 2 hours in the refrigerator, or 20 minutes in the freezer.
Roll out the dough on a marble slab or clean counter top, then line a buttered pie pan with it. Chill in the refrigerator while you make the filling.
Trim the leaves from about 6 stalks of rhubarb, then cut into one-inch pieces. Cut away any brown spots. Put the cut stalks and about a tablespoon of water in a heavy pot over very low heat. Cover and simmer, stirring often. Add a small amount of water if the mixture becomes too sticky. The cut stalks will melt into a rich red puree. Add a cup or more of sugar and continue to cook until the sugar is completely melted. Remove from the heat and grate the zest of 2 oranges into the puree.
When the mixture has cooled a bit, pour it into the piecrust, and bake for 30 to 40 minutes to brown the pastry. The tart, for that is what it really is, will be runny. You can serve it hot, but we prefer it at room temperature, or better yet, cold, for breakfast.