Preheat the oven to 170C/340F/Gas 3½
Wrap the whole garlic bulb in aluminium foil and place on a tray in the oven. Roast for 40-45 minutes, or until soft.
When the garlic is cool enough to handle, cut the top off of the bulb. Squeeze out all the soft garlic cloves and finely chop.
Pour the double cream into the bowl of a freestanding mixer and beat for 3-5 minutes, or until the mixture turns to solid fatty lumps with a liquid base.
Line a colander with a clean muslin cloth and place it over a bowl. Tip the mixture into the colander and wrap the muslin tightly around the cream. Squeeze out all the liquid so that you have a solid mass of butter in the muslin, then discard the liquid.
Tip the butter into a bowl, add the chopped garlic and a generous pinch of salt. Stir in the chopped parsley and mix once more.
Heat a large griddle pan and brush the bread with oil. Place on the griddle pan and cook each side for 1-2 minutes.
Spread the bread with the garlic butter and serve.