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1
Preheat oven to 400u00b0F Place potatoes in large bowl. Snip root end off each onion and cut each half into 3 sections. Add onions to potatoes, breaking up onions with your fingers to separate.
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2
Drizzle vegetables with oil, add thyme and toss well. Spread vegetables on baking sheet. Season with salt and pepper.
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3
Slice top third off head of garlic and place it on a piece of foil. Put 1/2 tablespoon butter on top of garlic and drizzle 2 teaspoons water onto foil. Fold sides of foil to enclose; place on pan with vegetables.
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4
Roast vegetables 45 minutes, then remove garlic. Continue roasting vegetables until tender, about 15 minutes more.
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5
In large saucepan, bring broth to a simmer over medium-low heat. Transfer half the roasted vegetables to food processor and add 1/2 cup hot broth. Process mixture until fairly smooth, then transfer to bowl. Repeat with remaining vegetables and a little more hot broth, squeezing garlic pulp out of husks into last batch. Stir all pureed mixture into broth in pan; keep pan over low heat.
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6
In small saucepan, melt remaining 2 tablespoons butter over medium-low heat. Add flour and cook, stirring constantly, 1 1/2 minutes. Stir in milk and cook, stirring often, until thickened, about 3 minutes. Stir in ladleful of soup and cook 1 minute.
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7
Add mixture back to soup. Cook over medium-low heat until heated through, stirring occasionally, about 10 minutes. Serve right away, or cool, refrigerate and reheat when needed.
