Preheat oven to 350°.
Remove white papery skin from garlic head (do not peel or separate the cloves). Wrap head in foil. Bake at 350° for 40 minutes; cool 10 minutes. Separate cloves; squeeze to extract garlic pulp. Discard skins.
Heat oil in a Dutch oven. Add garlic and onion; sauté until onion is tender. Add tomatoes and tomato paste; simmer 5 minutes. Add 2 1/2 cups chicken broth. Combine 1/2 cup broth and flour in a small bowl; stir with a whisk. Add flour mixture to tomato mixture. Bring to a boil; reduce heat, and simmer, uncovered, 30 minutes or until mixture thickens, stirring occasionally.
Place soup in batches in a blender or food processor; process until smooth. Return mixture to Dutch oven; stir in basil, half-and-half, and pepper.