Preheat oven to 350°F and butter a 9-inch round baking pan with high sides, or a 10-inch quiche dish, or whatever you have that will accommodate such a cake.
In a medium bowl, beat the butter and sugar until light and fluffy. Add the eggs and vanilla or lemon zest and beat until thick and pale yellow. Add the flour, baking powder and salt and beat on low speed or stir by hand just until combined.
Spread the batter in the pan, smoothing the top, and arrange the plum slices (or other fruit) over it in slightly overlapping concentric circles. Or whatever.
In a food processor, pulse together all the crumb ingredients until well blended. If you don't have a food processor, chop the walnuts and blend everything with a fork or your fingers.
Sprinkle the crumbs over the fruit, squeezing as you go to create big clumps, and bake in the middle of the oven for 50 minutes to an hour, or until the cake part is springy to the touch, or a tester comes out or or less clean (with some fruit on it).