Preheat oven to 450 . Toss cauliflower florets in 1/4 cup olive oil and roast on a sheet pan for 30 minutes, or until the edges are nicely browned.
In the meantime, heat 2 tbsp olive oil in a large stock pot over medium heat. Sauté onions, carrots, and celery until softened and beginning to brown. Add bouillon, water, bay, nutmeg, roasted cauliflower. Taste and add salt and pepper as needed. Bring to a boil, then blend with an immersion blender before serving.