Special equipment: 9-inch ovenproof pie dish (about 2-inches deep)
Preheat the oven to 375 degrees F, and slip in a baking sheet at the same time. Put the amaretti into a resealable plastic bag and bash with a rolling pin or similar, until reduced to coarse crumbs, then decant them into a bowl.
Melt the 2 tablespoons of butter in a large saucepan (that comes with a lid), add the prepared plums, sprinkle in the 2 tablespoons of sugar, add the lemon zest and juice, and shake the pan over the heat, cooking for 2 minutes without a lid and 2 further minutes with the lid on. These timings are based on having plums that are ripe; if the fruit is disappointingly unyielding, be prepared to cook for longer with the lid on, checking frequently. You may need to add the juice of the remaining half lemon - and more sugar - if cooking for much longer.
Pour the plums (with care--they're hot) into your pie dish and set to one side. Already the red skins will have made a gorgeous garnet gravy. Sprinkle in 2 tablespoons of your amaretti crumbs.
To make the crumble topping the easy way, put the flour and baking powder into the bowl of a freestanding mixer, shake to mix, then add the small, cold butter cubes and beat, not too fast, with the flat paddle until you have a mixture rather like large-flaked oatmeal. Or you can do this by hand, just by rubbing the butter into the flour with your fingers.
Add the sugar and mix with a fork, then tip in the rest of the amaretti crumbs and fork to mix again. Pour the mixture over the waiting fruit in its pie dish, making sure you cover right to the edges to stop too much leakage; although for me, some of the rich-hued syrup spurting out over the crumble topping is essential.
Place on the baking sheet in the oven and bake for about 30 minutes; by which time you should see some ruby bubbling at the edges, and the top will be scorched gold in places. If you can bear it, let this stand for 10 to 15 minutes before eating, with ice cream, whipped cream, or mascarpone.
Reprinted from Nigellissima. Copyright (c) 2012 by Nigella Lawson. Published by Clarkson Potter, a division of Random House LLC. All rights reserved.