Put the milk for the polenta in a saucepan together with 400 ml water, the bay leaf, onion and thyme if using. Bring to just below the boil then take off the heat and set aside to infuse for 20 minutes.
Strip the spring greens or kale leaves away from their tough stalks. Roll up the leaves and slice across them to produce thick ribbons. Steam for about 3 minutes ( or boil for 2) until tender. Squeeze or press the greens in the colander to remove as much water as you can. Toss with the knob of butter and some salt and pepper and keep warm.
For the polenta strain the infused milk and water into a clean pan. Bring to a simmer then add the polenta in a thin steady stream, stirring as you do. Stir until the mixture is smooth and let it return to a simmer.
Cook for just one minute and remove from the heat. Stir in the butter and season generously with at least a 1/4 tsp salt.
Scoop the soft polenta into warm bowls ( if it seems to thick loosen with a splash or two of hot water). Top with the crumbled cheese and put a heap of buttered greens alongside. Grind over some pepper and serve.