1. Preheat oven to 350ºF. Toss bread with 2 Tbsp. oil and 1/2 tsp. salt, spread in a single layer on a baking sheet, and bake until crisp, 12 to 15 minutes. Let cool.
2. Warm remaining 1 Tbsp. oil in a large saucepan over medium-high heat. Add garlic and cayenne and cook, stirring, until fragrant, about 1 minute. Stir in squash, chicken broth and 2 1/4 cups water, bring to a boil, lower heat, cover, and simmer until squash is soft, about 25 minutes. Puree in batches in a blender and season with salt.
3. Ladle soup into bowls, sprinkle with croutons, and serve.