Combine the flour, walnuts, and sugar in a large bowl. Add the butter and the egg yolk. Mix, either by hand or with an electric mixer, until crumbly.
Press 1 1/2 cups of the crumb mixture in an even layer into the bottom of a 9 1/2-inch springform or tart pan. Arrange the plums in the pan, skin-side down, to form a flower pattern; begin at the outside and work your way in.
Sprinkle the rest of the crumb mixture evenly over the plums. Bake the tart for 40 to 50 minutes, or until it's lightly browned and the plum juices are bubbling. Remove from the oven and cool for 10 minutes. Remove from the pan and transfer the tart to a flat plate. Serve warm or at room temperature.
Recipe courtesy of Ina Garten's The Barefoot Contessa Parties! (Clarkson Potter, 2001)