Make sure the plums are ripe enough - it makes all the difference!! (to ripen plums: place in paper bag - NOT plastic - do not refrigerate; check for softness; should ripen in a day or two. do not OVER-ripen.)
Preheat oven to 400. Spray pie plate or tart pan with cooking spray (or use false-bottom tart pan).
Put flour in mixing bowl, add nuts and brown sugar, mix to combine.
Add cold butter, egg yolk. Mix together to combine everything.
Take 3/4 of crumb mixture and press into bottom and sides of pan, leaving bottom nice and smooth. Save remaining mixture for step 5. Cut plums in half, remove pits. Cut into quarters (or sixths if very large) and put into crust, arranging them in a circular pattern.
Sprinkle remaining crust mixture on top of fruit.
Bake at 400 for 45 minutes, or until top crust is nicely browned.