Directions

1. Spray a 9-inch springform pan with cooking spray and set aside. Cut 4 tablespoons butter into cubes and place into the freezer.
   
2. Add 1 1/2 cups and 2 tablespoons flour to a mixing bowl and make a well in the center. Add the yeast and half of the milk as well as the 3 tablespoons of sugar. Mix the liquid ingredients with a spatula, incorporating just enough of the flour to make a wet paste. Let sit 15 minutes.
   
3. Stir in the remaining milk, egg yolk, 3 tablespoons of melted butter, pinch of salt, and lemon zest, and mix everything together well. Knead the dough on a lightly floured surface until smooth.
   
4. Shape the dough into a smooth ball and put in the prepared springform pan. Cover with a kitchen towel, and let rise in a warm place for 1 hour, until doubled.
   
5. Put the oats, 1/2 cup flour, brown sugar and 2 tablespoons sugar, cinnamon, and pinch of salt in the bowl of a food processor and pulse until the oats are broken up.
   
6. Add the frozen butter and another pinch of salt and process until the mixture first becomes granular, then begins to clump together. Use your fingers to smooth the yeasted dough across the bottom of the pan and about an inch rim up the sides. Place concentric rounds of prune plum wedges over the dough, within the rim, pushing them close together.
   
7. Strew 1 cup of streusel topping over the top and let the dough rise 20 minutes. Preheat the oven to 350F.
   
8. Bake the tart for 45-55 minutes, until the streusel topping is golden brown. Remove from the oven and let cool. Serve with whipped cream or ice cream.