Whisk together 1/4 cup of the sugar, the ascorbic acid, and 1 tablespoon of Signature Secrets or flour (or 1 1/2 teaspoons Instant ClearJel), then mix with the peaches and set aside. Roll the pie crust dough into a 14- to 15-inch circle to fit an 11- to 12-inch tart pan or baking dish. Place it into the pan.
Stir together the cream, the remaining sugar and thickener, the egg, nutmeg and vanilla; spread over the bottom of the crust. Place the peaches onto the cream filling, arranging them in concentric circles. Spoon any juice from the bowl over the peaches. Fold the edges of the crust in toward the center.
Bake the tart in a preheated 425°F oven for 20 minutes. Reduce the oven heat to 375°F; tent the edges of the tart with foil, and bake for an additional 30 to 35 minutes. Cool at least 20 minutes before serving. If pie won't be served right away, refrigerate it, and then bring to room temperature before serving. Just before serving, sprinkle with fresh berries and sugar. This is best served the same day.