Sauté Aromatics: In a large pot, heat the oil/butter over medium heat. Sauté the onion and garlic until soft (skip this if your leftovers already contain plenty of onion/garlic).
Combine & Simmer: Add your leftover roasted carrots, butternut squash, and brussels sprouts to the pot. Pour in the stock, covering the vegetables completely. Bring to a boil, then reduce heat and simmer for 5-10 minutes to heat through and meld flavors.
Blend: Use an immersion blender to puree the soup until smooth. If you want a textured soup, only blend half and leave the rest chunky. Note: If using a stand blender, be careful blending hot liquid—work in batches.
Finish: Stir in the optional cream/milk/beans if desired, and season with salt, pepper, and herbs to taste.
Serve: Garnish with extra roasted Brussels sprouts or pumpkin seeds.