Place flour, sugar, salt, and margarine in a food processor fitted with the knife attachment. Process until coarse crumbs form. Add water one tablespoon at a time until a dough forms. Shape dough into a disk, wrap in plastic wrap, and chill for one hour.

Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius). On a floured surface, using a floured rolling pin, roll dough into an 11-inch (28-cm) circle. Fit dough into a 9-inch (23-cm) tart pan with a removable bottom. Trim dough even with top of pan. Line crust with aluminum foil; fill with pie weights or dried beans. Bake crust until just set, 12-15 minutes. Transfer pan to a wire rack to cool. Remove foil and weights.

In a large skillet heat margarine and oil over medium heat. Add onions and shallots and cook — stirring frequently — until brown, 20-25 minutes.

Meanwhile, whisk together soy milk, eggs, egg yolk, and salt.

Spread pesto over prepared crust. Spoon onion mixture on top. Pour custard mixture over all. Bake tart until filling is set, 20-25 minutes. Transfer pan to a wire rack to cool slightly.