• Preheat oven to 400F/200C. 
• Cut the base off the cauliflower so that it sits more or less flat. Next, cut the cauliflower in half through the middle of the core, all the way through. Cut a slice around 1 - 1.5 in/2.5 - 4 cm thick from each half right the way through from each half so you get two flat 'steaks'. Keep the florets you have cut off for something else eg one of the cauliflower rice recipes listed above.
• Warm ½tbsp butter over a medium heat in a small to medium skillet/frying pan that fits both of the cauliflower steaks in lying flat. Add the cauliflower and cook a couple minutes on each side so that it becomes nicely browned. Add the lemon zest and some salt and pepper as it cooks. Add the additional butter in places that seem short on butter so that all of the edges being cooked are coated to help it brown. Turn as needed to brown on both sides.
• Transfer the pan to the oven and allow to cook for around 10 minutes, or possibly a little longer, until the cauliflower is completely tender.

For the salsa verde

• While the cauliflower is in the oven, finely chop the parsley, capers, anchovies and garlic together until they are very fine and mixed.
• Put all into a small bowl and add the mustard, lemon juice and olive oil. Mix well so they are well combined and serve over the cooked cauliflower steaks (you probably won't need all of it).