Combine peach nectar, sugar, lemon juice, and vanilla bean paste in a small saucepan over medium heat. Stir until sugar dissolves, about 2-3 minutes. Remove from heat and cool completely to room temperature, at least 15-20 minutes.
If using fresh peaches, peel by scoring an X on the bottom, boiling for 30 seconds, then transferring to an ice bath. Slice into 1/4-inch (6mm) uniform slices. If using frozen, thaw and pat very dry with paper towels.
Beat mascarpone on medium speed until smooth, about 1-2 minutes. Add powdered sugar, vanilla, and almond extract, mixing until combined. In a separate bowl, whip heavy cream to firm peaks, about 3-4 minutes. Gently fold whipped cream into mascarpone in three additions.
Quickly dip each ladyfinger into the cooled peach soak for 2-3 seconds per side. Do not oversoak. Dip immediately before layering.
Arrange a single layer of soaked ladyfingers in the bottom of a 9x13 inch baking dish. Spread half of the mascarpone cream evenly over the ladyfingers. Layer half of the prepared peach slices over the cream.
Dip the remaining ladyfingers and arrange them in a second layer over the peach slices. Spread the remaining mascarpone cream over the ladyfingers, smoothing to the edges. Press the reserved peach slices into the top cream in an attractive pattern.
Cover tightly with plastic wrap and refrigerate for a minimum of 4 hours, but overnight (8-12 hours) is strongly recommended for best results.
Just before serving, dust generously with freeze-dried peach powder or powdered sugar. For a brûléed topping, sprinkle peach slices with turbinado sugar and caramelize with a kitchen torch. Slice and serve chilled.