Combine the flour, sugar, baking powder and salt in the work bowl of a food processor. Pulse to combine. Scatter in the butter pieces and pulse until the butter pieces are about the size of peas.

Add the egg yolk and a tablespoon of ice water and pulse until the dough just comes together (it should clump when pressed in your fingers, but shouldn’t be wet). If the dough doesn’t come together, continue to add water, a tablespoon at a time and pulse quickly until it does (You may need up to 3 to 4 tablespoons of water all together).

Dump the dough onto the counter, press it into a disk and wrap in plastic. Let rest in the refrigerator for 1 hour. (The dough can also be made a day ahead, but let it sit out of the fridge for 10 minutes or so to soften a bit before trying to roll). Dust the dough with flour and roll out to about a 12-inch round.

Transfer to a 9-inch tart pan and press into the pan. Roll the rolling pin over the edges of the pan to trim any excess dough. Chill the tart shell 30 minutes. Meanwhile, preheat oven to 375 degrees. Line the chilled shell with parchment and fill with pie weights or dried beans. Bake on the middle rack until set, but not colored, about 15 minutes. Remove the parchment and weights and bake until the edges are just beginning to turn golden, about 5 minutes. Cool on a rack. Reduce oven temperature to 350 degrees.

Brush ¼ cup of the apricot jam over the bottom of the prebaked tart shell. Starting from the outside edge of the shell, arrange the plums in concentric circles until the shell is filled. Bake the tart 10 minutes, sprinkle the sugar over the plums, and return to the oven for an additional 25-30 minutes or longer, depending on the texture of the plums, until the pastry is nicely browned and the plums are well cooked.

Remove from the oven and cool.

Melt the remaining apricot jam in a small saucepan, stirring in the water and lemon juice. (Strain if the jam is lumpy). When the tart has cooled somewhat, brush the plums with the apricot glaze. Serve at room temperature.