Press butter pastry dough evenly over bottom and up sides of a 10-inch tart pan with removable rim.
Bake in a 300° oven until pale gold, about 20 minutes (about 15 minutes in a convection oven). Use hot or cool.
Rinse rhubarb, trim and discard dried ends, and cut stalks into 1-inch lengths. Put in a 10- to 12-inch frying pan and mix with 5 tablespoons sugar and 1/4 cup water. Let stand 10 minutes, then stir and set over medium-low heat. When water boils, turn rhubarb pieces over once and cook about 2 more minutes. Remove from heat.
Meanwhile, in a food processor or blender, whirl nuts to a fine powder. To processor, add remaining sugar and the butter, eggs, and almond extract. Whirl until well blended. Or put the ground almonds in a bowl, add remaining sugar and the butter, eggs, and almond extract, and beat with a mixer until well blended.
Pour almond mixture into pastry. With a fork or slotted spoon, lift rhubarb pieces from cooking liquid (save liquid for other uses) and arrange in a pattern on filling.
Bake in a 350° oven until filling, which rises around rhubarb, is golden brown and center is firm when pan is gently shaken, 35 to 50 minutes (25 to 40 minutes in a convection oven).
Let cool at least 15 minutes; remove pan rim to cut. Serve warm or cool.
Butter pastry: In a food processor or bowl, combine 1 1/3 cups all-purpose flour and 1/4 cup sugar. Add 1/2 cup (1/4 lb.) butter or margarine, in chunks. Whirl, or rub with your fingers, until fine crumbs form. Add 1 large egg yolk; whirl, or mix with a fork, until dough holds together. Firmly pat into a ball.