Preheat oven to roast at 425 degrees. Chop cauliflower, spread in one layer on baking sheet, drizzle and toss in olive oil and then salt generously. Roast until brown. Meanwhile, in the stockpot, melt 2 tbsp butter, then add chopped leeks, chopped celery, sprigs of thyme and parsley. Add salt and garlic and cook on medium heat to sweat leeks until translucent. Add chicken stock, continue to cook on medium heat for 20 minutes. Remove from heat, and carefully pour into Vitamix. Blend on high until smooth. Return to stockpot and add half and half, heavy cream, olive oil, and additional salt to taste. Blend until smooth. Top with croutons and green onion and serve.
Croutons: Dice loaf of bread into crouton-sized pieces and add to large bowl. Toss with olive oil, melted butter, parmesan cheese, thyme, and garlic. Spread into one layer on large baking sheet and roast until golden brown at 425 degrees.