Crush the juniper berries thoroughly using a pestle and mortar (or bash with a rolling pin, in a sealed plastic bag). Tip into a pan with the gin and milk and put over a high heat. Bring to the boil and bubble for 3 minutes, stirring now and again. Remove from heat, season with fine sea salt and freshly ground black pepper. Set aside.
Discard the tatty outer leaves of the cabbage, then cut it into eighths. Cut out and discard the hard core from each wedge then slice crossways into ruler-wide strips. Halve the leek lengthways, then cut into 1cm/½in wide chunks. Tip the vegetables into a colander and rinse with water.
Put the butter into a very large pan or wok and add the vegetables, plus residual water. Sprinkle on the bouillon powder and season well. Cover the pan and sit over a medium heat.
Leave the lid on for the first 6 minutes but remove every 2 minutes to toss the cabbage. Remove the lid and cook and toss the cabbage for 4-6 minutes more. Tip into a colander and drain well. Return to the pan and keep warm.
Replace the small pan over a low heat, whisk in the fromage frais and season with black pepper. Gently warm the sauce (it will split if you boil it), then pour over the cabbage and toss.