One of my online friends calls this technique hypercooked vegetables – long cooking so familiar in the traditional Italian and Greek cuisines. The result is a surprising lusciousness, intensity of flavour, and an almost stickiness. They are deeply flavoured and a little tart. I have made this dish with cabbage and with beetroot greens, but I am sure it would work with any leafy greens that do not collapse immediately on heat (eg most of the salad greens would be unsuitable).
You will find it difficult to stop diving into the cooking pot once these have collapsed down into their jammy texture. But if you do leave some, serve as a side dish, or over rice or any other grain, lentil or bean (freekeh, couscous, white beans, burghul, red rice, etc), turn into a soup with a handful of the one of the tiniest soup pastas, orzo pasta or rice, or just ladle it over thick slices of toast with a drizzle of olive oil. I have also cooked turnips, diced, and added to these beetroot greens. I sometimes add sultanas to counterpoint the tartness.
The mustard seeds and cumin that I added this time are purely optional.
Do try on a lazy Sunday afternoon, when you have time to let the greens collapse and intensify.