Cut 6-inch square piece of aluminum foil. Slice 1/2-inch off top of garlic and set head on foil. Drizzle garlic with 1 teaspoon olive oil; wrap in foil and bake for 40 minutes, until soft. Remove and set aside to cool.
Season chicken parts with garlic salt, oregano and 1/4 teaspoon pepper. In large soup pot over medium-high heat, warm 2 tablespoons olive oil. Add chicken and sauté until browned, about 5 minutes per side. Remove chicken and set aside. Pour off all but 1 tablespoon of liquid. Add bell peppers and sauté for 2 minutes. Open foil and squeeze garlic out of skin, adding to pot.
Stir in tomatoes with juice and broth; replace chicken. Bring to simmer over high heat; reduce to low and let stew for 15 minutes. While stew is simmering, peel pearl onions and potatoes. Cut potatoes into 1-inch chunks and cut corn into 2-inch chunks; slice squash. Stir onions, potatoes, corn, squash and sage into stew; cook for 15 more minutes. Season with salt and remaining pepper; serve immediately in wide, shallow bowls.