Brown three lamb shanks all over in a very hot skillet (I used rendered lamb fat from a previous cook but a small amount of any fat with a high smoke point is fine), then remove from the pan and arrange in a slow-cooker crock. Lower the heat a bit and saute a mirepoix (i.e. roughly chopped) of one leek, three or four carrots, a few stalks of celery and a handful of mushrooms, using the residual fat from the shanks plus a glug of olive oil, then add a few chopped garlic cloves and cook briefly to take the raw edge off. Add a cup of unsoaked puy lentils, cook briefly (mainly to mix) and then deglaze with a good slosh of white wine. Transfer the contents to the crock with the lamb shanks, then rinse the pan out with three cups of water – pour the three cups straight from the pan into the crock pot too. Add a generous pinch of salt and your choice of “hard” herbs (I used thyme and bay leaf, but rosemary or sage would also be fine) and a grind of pepper, and cook on low for about eight hours.
After the cooking time is finished, remove the shanks carefully (if you can without them completely disintegrating), stir in a whole tin of beans (e.g. cannelini or borlotti), liquid and all (it’s a natural thickener), and cook on high for half an hour or so. Warm the shanks through in the oven or microwave, then place on top of a bowl of the lentil stew to serve.