Roast a whole garlic bulb for 1 hour or drizzle individual cloves with olive oil and salt and bake at 350 until soft (about 15 minutes).
Bring a large pot of salted water to a boil. Add the potatoes and the cauliflower (I did them at the same time) and boil until knife-tender (until you can poke with a knife and it easily slides out). My cauliflower took about 15 minutes, my potatoes about 25-35.
In a food processor, puree the cauliflower with roasted garlic, 2 tablespoons of olive oil, rosemary and a few generous pinches of salt and pepper. Taste and adjust seasonings. (Tip: this mixture can be over-flavored because it'll get mixed with the potatoes later).
When the potatoes are soft, remove them from the pot. Peel them (careful, they’re hot) and use a food mill, ricer, or masher, to mash them. Add 2 more tablespoons of olive oil, then stir the cauliflower puree into the bowl with the potatoes. Stir, in a folding motion until smooth and creamy. If necessary, add ¼ cup (or so) of hot starchy potato water to help it out. Taste and adjust seasonings, adding more olive oil, salt, pepper, and/or rosemary to taste.
Serve hot. Mine actually reheated great too - store in the fridge, microwave the next day.
I'd advise against using raw garlic here. (We tested it, and it's just too strong). If you don't feel like roasting the garlic, make garlic-oil instead. Smash a few cloves into your ¼ cup of olive oil. Let it infuse for at least a few minutes and remove garlic pieces before using.