Pre-heat oven to 425 degrees.
Cut cauliflower florets away from stems and place in a single layer on baking sheet. Place garlic cloves, with peels still on, on the sheet with the cauliflower and drizzle with 2 Tbsp olive oil.
Bake cauliflower and garlic with peels on 30-35 minutes minutes; take garlic out after 20 minutes.
Sauté onion in 2 Tbsp olive oil in a large stockpot or dutch oven until softened.
Add vegetable broth, cauliflower, and garlic cloves and bring to a boil. Lower to simmer for about 8 minutes.
While cauliflower is simmering, in a separate medium bowl, whisk together milk, flour, and garlic powder. Stir mixture into cauliflower mixture.
Remove from heat and puree using an immersion blender. (If you don’t have an immersion blender, you can puree soup in batches in blender and then return to pot.) Once pureed, place stockpot back on burner over medium low heat.
Gently stir in grated cheese until melted.
Serve in individual bowls and top with croutons.