Cut the top off the garlic and wrap it in foil leaving the top cut edge exposed. Drizzle on some olive oil and bake for 45 minutes or until the garlic is soft and caramelized. Remove from the oven and let cool a bit.
Meanwhile peel the potatoes, cut them into cubes, and put them into a medium sized pot. Cover them with water and add a big pinch of salt. Bring them to a boil over high heat, then lower the heat to medium and simmer until they are tender, about 20 minutes. Drain them well.
While the potatoes are cooking, warm the cream and rosemary in a small pot over low heat. When you are ready to mash, strain out the rosemary.
When the potatoes are dry but still hot, put them through a ricer into a large bowl. Squeeze in the garlic and add some of the warm cream. Mix well, then add the butter and mix again. Add more cream if they are too dry. Taste and adjust seasoning with salt and pepper. Serve immediately.