Step 1

Prepare the Oatmeal Piecrust: Stir together oat and all-purpose flours, brown sugar, and salt in a large bowl. Using a pastry cutter, cut cubed butter and shortening into flour mixture until it resembles coarse meal (pea-size bits with a few larger bits of fat are okay). Drizzle in cold water 1 tablespoon at a time, stirring until dough begins to form large clumps. Transfer to a floured work surface. Dough should come together easily and should not feel overly sticky. Using floured hands, fold dough until it's combined (there should be no dry bits visible) and fats are speckled throughout. Shape into a ball; flatten into a 1-inch-thick disk using your hands. Wrap tightly in plastic wrap. Chill until firm, at least 2 hours or up to 5 days.

Step 2

Preheat oven to 375°F. Unwrap dough roll into a 13-inch round on a lightly floured work surface. Transfer to an 11-inch tart pan, pressing into bottom and up sides of pan. Coat a piece of parchment paper with cooking spray; place, greased side down, on crust. Fill with pie weights. Bake 20 minutes. Remove from oven. Carefully remove parchment and pie weights. Return to oven; bake at 375°F until crust is set, about 10 minutes. Remove from oven. Cool completely, about 1 hour.

Step 3

While Oatmeal Piecrust bakes, prepare the Orange Custard: Whisk together egg yolks, eggs, whipping cream, granulated sugar, drink mix, and salt in a large bowl until well combined. Let mixture stand until room temperature, about 1 hour, 30 minutes.

Step 4

Fill a large baking pan with water to a depth of 1 inch. Place on bottom oven rack. Preheat oven to 300°F. Pour Orange Custard into cooled Oatmeal Piecrust. Bake until center of filling is set, 45 to 55 minutes. Turn oven off; let tart cool in oven with oven door ajar for 20 minutes. Transfer to a wire rack; cool to room temperature, 1 hour. Chill, uncovered, until cold, about 4 hours or up to overnight (12 hours).

Step 5

Prepare the Swiss Ginger Meringue: Beat egg whites with a stand mixer fitted with a whisk attachment on medium-high speed until frothy, about 1 minute. Add cream of tartar; beat 1 minute. Beat in cornstarch, ground ginger, and salt until soft peaks form, about 2 minutes. Turn mixer off.

Step 6

Cook sugar and corn syrup in a small saucepan over high, stirring occasionally, until a thermometer registers 240°F, 3 to 4 minutes. Reduce mixer speed to low, and slowly drizzle about 2 tablespoons syrup mixture into egg white mixture. (If you add too much syrup at once, the whites will scramble.) Slowly drizzle in remaining syrup mixture. Increase mixer speed to medium-high, and beat until meringue is stiff and glossy, about 6 minutes. Spread meringue evenly on top of cooled tart. Using a kitchen torch, lightly toast meringue. If desired, garnish with sliced seasonal fruit. Serve immediately, or store, covered, in refrigerator up to 1 week.