Wrap entire garlic bulb tightly in foil; place garlic and potatoes in oven. (You do not need to put them on a pan; they can go right on an oven rack.).
Bake garlic until soft when squeezed, about 45 minutes; remove from oven and let cool. Continue baking potatoes until tender when pierced, about 15 minutes more; let potatoes stand until cool enough to handle.
Meanwhile, cook bacon in a large nonstick skillet over medium-high heat until browned, about 6 minutes.
Place bacon on paper towels to drain off any fat; chop bacon.
Unwrap garlic and squeeze pulp from bulb with hands into a large saucepan.
Peel potatoes and add to saucepan; mash with a potato masher until smooth.
Gradually stir in broth, thyme, salt and pepper until blended; place saucepan over medium heat and cook until hot, stirring occasionally, about 5 to 10 minutes.
Spoon about 1 1/3 cups of soup into each of 6 soup bowls.
Top each with 1 tablespoon of sour cream, 1 tablespoon of cheese, 1 rounded tablespoon of bacon and 1 tablespoon of scallions.