Place spinach, water, bouillon, onion, and garlic powder in a large pot over medium-high heat. Bring to a boil, then reduce heat to medium-low and simmer until onion is tender.
Meanwhile, melt the butter in a small saucepan and whisk in the flour until smooth. Cook for 2 minutes. Slowly stir in half-and-half; mix until smooth. Pour the cream mixture into the spinach soup, and simmer until thickened, about 10 minutes. Season to taste with salt and pepper.