Mashed Potatoes with Roasted Garlic and Rosemary

2 whole heads garlic
2 lbs peeled cubed potatoes (Yukon gold recommended)
1 c chopped onion
2 Tbsp plan yogurt
1 tsp dried rosemary, chopped
1/2 tsp salt
1/4 tsp freshly ground black pepper

Remove white papery skin from garlic heads -- do not peel or separate cloves. Wrap each head separately in foil. Bake at 350 degrees for 1 hour; cool 10 minutes. Separate cloves; squeeze to extract garlic pulp. Discard skins.

Place potatoes and onion in a saucepan; cover with water and bring to a boil. Cover, reduce heat and simmer 15 minutes or until potatoes are tender. Drain in a colander over a bowl, reserving 1/4 c cooking water.

Combine garlic, potatoes, reserved cooking liquid, and remaining ingredients; mash.

NOTES
From Cooking Light.
Untried.
I think I'd probably saute the onions instead of boiling with the potatoes.