Preheat oven 450 degrees F. In a mixing bowl toss cauliflower florets, olive oil, salt, black pepper, and red pepper flakes. Evenly distribute onto rimmed baking sheet. Bake 20-25 minutes or until nicely roasted, stirring once.
In a large stock pot, add 2 tablespoons olive oil and heat over medium high. Once oil is almost smoking add leeks, saute 2-3 minutes until fragrant and browned. Add garlic and cook 1 minute; stirring constantly. To deglaze pan: add white wine, while stirring and scraping up bits off the bottom of pot. Add mushrooms and cook 2 minutes, stirring.
Add chicken broth and reduce heat to medium low. Allow to simmer until cauliflower is roasted. Add cauliflower to pot and stir. Blend half of vegetable mixture in high efficiency blender until creamy and smooth. (I like to blend twice to achieve a really smooth mouth feel) Return to stock pot and simmer over low. In a medium size sauce pan, over medium high heat; add 2 tablespoons butter and melt. Whisk in flour slowly and cook, stirring constantly for 1 minute. Slowly whisk in both milks, bring to simmer then add 2 cups cheese; stir until melted. Add to stock pot; stir to incorporate.
Grilled Cheese Croutons:
Using the remaining butter, spread butter on one side of each slice of bread. Place one slice, butter side down on hot skillet, add 1/2 cup shredded cheese, top with additional slice bread- butter side up. Cook 2 minutes, turn and repeat. Remove from heat and cut into 1/2 inch size cubes. Ladle soup into serving bowlssprinkle with additional 1/2 cup shredded cheese and croutons to garnish.