Heat the oven to 400 degrees F. Slice the top off the garlic head, drizzle with the olive oil, wrap it in foil and roast for 30 to 35 minutes until the cloves are soft and golden.
Put the potatoes, thyme and the tablespoon of salt in a large pot, cover with cold water and bring to a boil. Cook for 10 minutes.
Add the cauliflower florets and cook for 10 to 12 more minutes until everything is fork-tender.
Drain well, pull out the thyme stems and return the vegetables to the hot pot. Let them sit uncovered for 2 minutes so the steam escapes.
Squeeze the roasted garlic cloves into the pot. Add the butter, Greek yogurt, olive oil, salt, pepper and lemon juice. Mash by hand or whip with a hand mixer until smooth.
Taste for salt, fold in most of the chives and serve with the rest on top.
