Peel and cube 2 of the potatoes. Cube remaining 2 potatoes, leaving the skin on. Place peeled potatoes on one side of a 9x9x2-inch baking pan and the unpeeled potatoes on the other side. Sprinkle onion and snipped rosemary evenly over all. Peel away the dry outer leaves of skin from bulb of garlic. Leave skins of cloves intact. Snip off the pointed top with scissors, leaving the bulb inact, but exposing the cloves. Place the garlic bulb, cut side up, on top of the potatoes. Drizzle olive oil over all. Bake, covered, in a 400 degree F oven about 50 minutes or until garlic cloves feel soft and potatoes are tender. Cool slightly.
Squeeze the garlic bulb to remove the paste from the cloves. In a blender container or food processor bowl, combine the garlic paste, peeled potatoes, about half the onion, half of the chicken broth, all of the flour, and pepper. Cover and blend until nearly smooth.
Pour the pureed mixture into a medium saucepan. Stir in the unpeeled cooked potato, remaining onion and broth, and half-and-half. Cook and stir over medium heat until slightly thickened and bubbly. Cook and stir for 1 minute more. If necessary, stir in additional half-and-half to make the soup the desired consistency. To serve, ladle into soup bowls. If desired, top each serving with Homemade Croutons and a sprig of rosemary. Makes 5 cups (4 side-dish servings).
Spread 2 cups cubed sourdough or rye bread in a single layer in a shallow baking dish. Stir together 2 tablespoons melted margarine or butter and 1/2 teaspoon Italian seasoning, crushed; pour over bread cubes. Bake in a 400 degree F. oven for 5 minutes. Stir; bake 5 to 10 minutes more or until crisp and brown. Makes 2 cups.