Thinly slice the onion. Cut the beans into 3 cm pieces and shred the cabbage.
Make the gremolata. Grate the lemon into a small bowl and mix it with the chopped parsley and minced garlic. Set aside.
Heat the oil in a large, shallow dish and gently fry the onion for 5 minutes. Add the greens, water and stock powder. Put a lid on and simmer for 5 minutes.
Meanwhile, cut the monkfish flesh into 3cm cubes or slices.
Put these on top of the sautéed green, then squeeze the lemon all over the dish and season the fish with salt and pepper.
Put a lid on and steam over medium heat for about 3 minutes. Don’t overcook - the fish will be chewy!
Scatter the gremolata on top and serve.