Preheat oven to 425°F. Line a baking sheet with parchment paper and spread cauliflower and garlic over. Toss with 1 tsp olive oil and roast in preheated oven for 20-22 minutes or until browned. Turn once during baking. Remove and set aside.
Meanwhile, in a large pot or dutch oven, heat 1 tbsp olive oil over medium heat. Add onion and sauté until soft and translucent. Add cauliflower, garlic, and stock and bring to a boil. Reduce heat and simmer, covered, for 20 minutes.
Add milk and season with salt and pepper. Purée using an immersion blender, or in batches in a regular blender, until desired consistency is reached.