Discard the cauliflower leaves and stem and cut into florets, reserving all the tiny florets and crumbled cauliflower from the board to use for your topping. Place the larger florets in a roasting dish along with the oil, seasoning, rosemary and garlic, and roast in the oven for 20 minutes until the cauliflower is starting to char.
Once roasted, remove any large sprigs of rosemary, squeeze the softened garlic out of its skin and set aside.
In a large pan, melt the butter over a medium heat before adding the leeks. Cook for 4-5 minutes until the leeks begin to soften but are not yet coloured. Add the tray of roasted cauliflower, garlic and potatoes to the pan and pour in the stock.
Bring the pan to the boil then turn down the heat to medium-low and allow to simmer very gently until the potatoes are cooked through – test with a sharp knife.
Take the pan off the heat, grate in the Cheddar and pour in the cream, stirring well to combine. Using a stick blender, blitz the soup until it’s silky smooth. Return to the hob over a low heat to warm through while you make the topping.
Place the reserved small piece of cauliflower on a baking tray and roast in the oven for 10 minutes. Tear the bread into 2cm pieces and coat in the oil. Add to the tray and cook for a further 2 minutes. Give the tray a shake and sprinkle in the seeds and salt. Return to the oven for a final 2-3 minutes until the bread is golden and toasted.
Serve the soup in generous bowls and top with the toasted bread and seed mix.
