Heat the butter in a pot over medium heat. Sauté the onions, garlic, and chicken until no longer pink and the onions start to soften. Stir in the flour and cook for 1-2 minutes.
Stir the broth, broccoli, and scallions. Season with salt and pepper. Cover with a lid and bring to a simmer for about 20 minutes. Stirring occasionally.
Preheat the oven broiler on low. Mix the bread, oil, garlic powder, parmesan, salt and pepper in a bowl. Lay out on a foil-lined baking sheet and place under the broiler until toasted golden brown, mixing the croutons around every couple minutes so all sides get toasted. Then set aside.
Stir the milk and cheese into the soup until it melts completely. Taste and adjust the seasoning with salt if needed.
Garnish with croutons, scallions, and shredded cheese.