Preheat oven to 180C/350F.
Chop off the top of your garlic bulb to expose the cloves. Place your cauliflower, shallots, garlic, rosemary onto a large baking sheet, in a single layer so the veggies roast evenly. Drizzle generously with olive oil and season with salt and pepper. Bake for 45min-1hr or until the veggies are soft and golden. Reserve some roasted cauliflower for garnish.
In a pot on medium-high heat, melt a knob of butter. Add in the leeks and sweat it until translucent.
Add in the diced potatoes, roasted veggies and stock. Bring to a simmer and cook until the potatoes are cooked.
Squeeze in the roasted garlic and blend the soup until smooth (you can either use a stick blender or a standing blender to do this).
Return soup back into the pot and add in the cream and lemon juice. You can add more water here if you prefer a thinner soup consistency. Season to taste. Garnish with reserved cauliflower and serve with grilled cheese.