Wash the plums, pat them dry, remove their stones and cut them into small pieces. Put them in a heavy-bottomed frying pan with the sugar and the lemon juice. Cook for 10-15 minutes, stirring occasionally, until the pieces of plum are soft and a syrup has almost formed. Remove from the heat and leave to cool down completely. In the meanwhile prepare the crumble: put the softened butter (taken out of the fridge at least 20 minutes before) cut in pieces, the sugar, the grated lemon zest and a pinch of salt in a bowl. First beat this mixture with an electric hand mixer, then add the sifted plain flour and the almond flour, and continue to stir with a wooden spoon until you have a coarse, crumbly mixture. Tip it out onto your work surface and work with your hands to for a ball of dough. Cover with cling film and leave in the fridge to rest for 30 minutes. When this time has elapsed, pour the cold plums into the bottom of an oven dish (approx. 15 x 25 cm) with their syrup, spread them out over the entire surface, and cover with the dough, “crumbled” but not into overly small pieces. Sprinkle with the flaked almonds and bake in a preheated oven, at 170 °C, for roughly 40 minutes. Remove from the oven and leave to cool before serving. You can use any seasonal fruit to prepare this dessert; apples, peaches, strawberries, apricots, cherries... and you can also enhance the crumble according to taste, with plain chocolate chips, crumbled amaretti biscuits, coarsely chopped hazelnuts... it’s always delicious!