Preheat the oven to 300°.

Prepare the pecans: In a small bowl, lightly beat the ½ egg white until frothy. Add the pecan halves and lightly coat them in the egg white. Remove to a plate and sprinkle with the sugar. Spread the nuts in the oven, shaking the pan occasionally and watching carefully, until golden, about 5 minutes. Remove from the oven and let cool. Set aside 16 pecan pieces for decoration.

Prepare the sweet potato mixture: Place the potatoes in a large saucepan with enough water to cover and bring to a boil over high heat. Cook for 15 minutes, or until fork-tender. Drain. In a mixing bowl, whip the potatoes and spices with an electric mixer until slightly lumpy or completely smooth. Cool in the refrigerator for 20 minutes. In a large bowl, using an electric mixer, beat the egg whites until frothy. Gradually add the 1/3 cup sugar and continue beating until stiff and glossy. A little at time, fold the meringue into the potato mixture. Fold in the sugared pecans. Spoon the mixture into the prebaked tart shell and smooth the top. Decorate with the reserved pecan pieces and the crystallized violets. Serve lightly chilled with unsweetened whipped cream.