In a medium bowl, stir together the flour, salt and sugar. Cut in margarine until the mixture resembles breadcrumbs. Stir in just enough water to hold together. Roll out on a lightly floured surface to 1/8 inch thick. Use dough to line 4 tartlet pans. Prick with a fork, and refrigerate until needed.
Heat the oil in a skillet over medium heat. Add watercress, garlic and onions. Cook and stir for about 2 minutes, or until fragrant. Remove from the heat and stir in the Parmesan cheese, yogurt and cayenne pepper. Spoon into the chilled tart pans.
Bake for 20 minutes in the preheated oven, or until filling is firm. Serve warm.