Depending on the size of your cauliflower you should be able to get 2-3 steaks out of it. Allow them to be 1-2cm thick, slicing from the top of the cauliflower to the stalk. Break the rest of the cauliflower into florets and chop up the leaves.
Place the steaks in a roasting tray with the rest of the cauliflower florets and leaves, drizzle over the oil and roast at 200 °C for 40 mins.
While this is roasting blend the pesto ingredients together.
Rinse the lentils add a drizzle over olive oil, salt and half the lemon juice. Mix the rest of the lemon with the tahini and yogurt