1. In a food processor or bowl, combine flour and 1/4 cup sugar. Add 1/2 cup butter, cut into chunks. Whirl or rub in with your fingers until fine crumbs form. Add egg yolk; whirl or mix with a fork until dough holds together. Firmly pat into a ball.
2. Press dough evenly over bottom and up sides of a 10- to 11-inch tart pan with removable rim.
3. Bake in a 300° regular or convection oven until golden, 25 to 30 minutes.
4. Meanwhile, in food processor (no need to wash) or a bowl with a mixer on medium-high speed, whirl or beat remaining 2 tablespoons sugar, 1 tablespoon butter, egg white, and almond paste until smooth. Gently spread over bottom of tart crust.
5. Return to oven and bake until crust is deep golden brown and almond paste mixture feels dry to touch, about 15 minutes. Let cool on a rack.
6. Arrange nectarines on almond filling in concentric circles around outside of tart, strawberries in the middle.
7. Heat jam in a microwave-safe bowl in a microwave oven on full power (100%) until melted, about 1 minute, stirring after 30 seconds. Brush evenly over fruit.
8. Chill until jam is set, about 1 hour, or chill airtight up to 6 hours. Remove pan rim and cut tart into wedges.