Bake nut shortbread crusts (set individual crusts in a 12- by 15-inch pan) in a 350 oven (or a 325 convection oven) until golden at edges, 20 to 25 minutes.
While crusts bake, make nut sauce in a 10- to 12-inch frying pan. Over medium-high heat, shake sugar in pan until melted and amber-colored, 4 to 6 minutes.
Add whipping cream and stir until caramelized sugar melts, 1 to 2 minutes. Stir in pecans and lemon peel; remove from heat.
Peel peaches, if desired.
Cut into halves, pit, then cut into 1/2- to 3/4-inch-wide slices. Arrange peach slices, slightly overlapping, in hot crusts or crust. Save extra fruit for other uses.
Spoon nut sauce equally over fruit; if sauce has thickened too much to spoon, stir and heat until fluid.
Bake in a 350 oven (or a 325 convection oven) until sauce is bubbling and crust is deep golden, 20 to 24 minutes for individual tarts, 50 to 55 minutes for a large tart.
Cool in pan or pans on a rack at least 30 minutes. If making ahead, chill airtight up to 1 day.
Remove pan rims.
Serve with ice cream.
Nut shortbread crusts: In a food processor, mince 1/3 cup pecans with 1 1/4 cups all-purpose flour and 1/4 cup sugar.
Add 1/2 cup (1/4 lb.) butter or margarine, in chunks, and whirl until fine crumbs form.
Add 1 large egg yolk and whirl until dough holds together. (Or mince nuts with a knife, then put in a bowl. Rub in flour, butter, and sugar until fine crumbs form; then, with a fork, mix in yolk. Pat into a ball.) In 6 tart pans (4 1/2 in. wide) with removable rims, or an 11-inch tart pan, press dough evenly over bottom and up sides, flush with rims.
