Preheat oven to 425 degrees F.
Cut cauliflower into small florets. Place about ⅔ of the florets on a sheet pan and toss with 1 tablespoon of olive oil, oregano, salt, and pepper. Roast for 20 minutes or until charred, flipping cauliflower over halfway through. Remove from heat and keep warm.
Pour stock in a medium sauce pot. Add rosemary and 2 cloves garlic and bring to a boil over high heat. Add remaining cauliflower and bring to a boil. Cover pot, reduce heat to low, and continue to simmer for 20 minutes or until cauliflower is very tender. Remove cauliflower and garlic with a slotted spoon and puree in blender until smooth. If cauliflower is too thick, add 1-2 tablespoons of broth. Add 2 tablespoons butter and blend until smooth. Season with salt and pepper.
Melt remaining 2 tablespoons butter in a small saute pan over medium heat. Continue to simmer until butter turns golden brown and smells nutty. Add panko, tossing to coat, and cook until golden brown and toasted. Remove from heat and stir in grated parmesan and black pepper.
Toast sourdough bread. Rub garlic clove on bread and spread cauliflower puree. Top with roasted cauliflower and sprinkle breadcrumbs, hazelnuts, and chives on top. Drizzle extra virgin olive oil and finish with a squeeze of lemon. Serve immediately.