Preheat oven to 375 degrees F.
Place garlic heads on aluminum foil and lightly spray each head with 2 sprays olive oil cooking spray. Wrap aluminum foil around the garlic to form a tight package. Roast for 45 minutes. Remove from the oven and place whole heads of garlic in a food mill. (Adding the whole garlic heads eliminates the step of removing the pulp from the cloves because the food mill squeezes the pulp out.) Process the garlic until smooth. Transfer garlic pulp to a bowl. Add the fresh herbs and sour cream and stir until completely blended. Slowly add buttermilk, adding just enough to give the dressing a consistency that is easy to pour. Taste and adjust seasoning with salt, pepper and lemon juice. Chill for at least 30 minutes to allow flavors to meld.
Just before serving, toss salad greens with dressing.