Peel the potatoes and cut them into 1-inch cubes. Cut the cauliflower into florets about the same size.

Put the potatoes and whole garlic cloves in a large pot, cover with cold water by an inch, and add a big pinch of salt. Bring to a boil and cook 10 minutes.

Add the cauliflower on top, cover, and cook 8 to 10 minutes more, until a fork slides into both the potato and the cauliflower with no resistance.

Drain in a colander and let it sit 2 minutes. Return everything to the hot, empty pot over low heat for 1 minute, shaking once, so the steam escapes and the cauliflower dries out.

Mash with a hand masher or press through a ricer. Do not use a blender or food processor, which turns potatoes gluey.

Add the butter and stir until it melts. Stir in the Greek yogurt, then the milk a splash at a time until it is as loose as you like. Season with the salt and pepper.

Taste and adjust the salt. Top with chives and serve warm.