Put 150ml of olive oil in a blender with 100ml of water, 10g of dill fronds and 15g of parsley leaves, then process to a vivid green dressing. Add a little salt and pour into a large bowl.
Cut a spring cabbage, or 2 if they are small, into slices about 2cm in thickness. Wash thoroughly, then toss them gently in the herb dressing and set aside.
Peel 750g of sweet potatoes, then cut into large pieces. Pack into a steamer basket or colander and place over a pan of boiling water. Cover tightly with a lid and leave them to steam for 25 minutes, until you can easily pierce them with a skewer.
Set the oven at 200C/gas mark 6, then line an oven tray or grill pan with kitchen foil. Place the slices of cabbage on the foil side by side, reserving any extra dressing that may be in the bowl. Dot 65g of butter between them, then bake for 20 minutes, until tender to the point of a knife. Baste them a couple of times as they cook.
While the cabbage is baking, tip the sweet potato into a bowl and mash to a smooth, thick purée with a masher or fork, then season generously. Divide the sweet potato mash between 2 warm plates, then remove the baked cabbage from the oven, trickle over any remaining dressing and serve. Enough for 2
Baste the slices of cabbage as they bake with any dressing or butter in the oven tray (and with a little more melted butter if necessary), and test every few minutes. You ideally want the outer leaves to be green-gold, lightly crisp at the edges and the stalks to be tender.
Use other root vegetables for the mash, including swede or large, maincrop carrots. If you didnt want to bake the cabbage, you can sauté it in a little oil and butter instead, basting it as it cooks.