In a large soup pot heat the olive oil and sauté the onion and the garlic for 2-3 minutes. Stir in the carrots and the celery and sauté for an additional 2-3 minutes. Stir in the cauliflower. Add the vegetable broth, 2 cups water, salt, and the pepper, stir together. Bring to a boil, then reduce heat and allow to simmer for 15 minutes.

In a medium saucepan over medium heat,  melt the 3 tablespoons of butter, stir in the flour until smooth. Gradually add the milk, stirring constantly. Continue to cook over medium heat for 1-2 minutes. Stir the flour mixture into the simmering cauliflower soup and simmer for another 15 to 20 minutes or until desired tenderness. Stir in the cheese.