A delicate custard holds slices of sweet Bosc pear and jewel-like cranberries in this elegant tart. Most of the time needed is hands off, and the press-in crust means no rolling, so this gorgeous dessert is much easier than it might appear.
Preheat oven to 350 degrees. Bring butter, sugar, salt, and2 tablespoons water to a boil in a saucepan. Reduce heat to medium and continue boiling, stirring occasionally, until mixture has a nutty aroma and turns golden brown, 7 to 9 minutes. Remove from heat. Stir in vanilla, then flour and oats, until no dry flour remains and a dough forms.
Transfer to a fluted 9-inch tart pan. Let stand until cool enough to handle, about 5 minutes, then press evenly into bottom and up sides of pan. Line with parchment and fill with pie weights or dried beans. Bake until crust is dry on bottom and darkened slightly, 20 to 25 minutes. Remove pie weights and parchment and continue baking until golden, about 5 minutes more. Let cool completely.
Meanwhile, in a bowl, whisk together eggs, 2/3 cup sugar, vanilla, and salt. Add butter and flour; whisk to combine. In another bowl, toss cranberries with remaining 3 tablespoons sugar. Scatter half of cranberries in bottom of crust. Top with pears, arranging them in a single overlapping layer. Scatter remaining cranberries over pears; transfer pan to a parchment-lined rimmed baking sheet. Slowly pour egg mixture evenly over fruits.
Bake until pears are easily pierced with the tip of a knife and filling is puffed and set, 45 to 55 minutes. Transfer tart in pan to a wire rack and let cool completely before slicing and serving.