If you are making the soup completely from scratch, begin by starting to make a pumpkin purée (or a butternut squash purée).
Caramelize the onion
Peel and cut the onion in half and slice into thin half-moons to get thin ribbons of onions to caramelize.
Pour the oil into a large frying pan and warm it at medium to high heat. Add the onion ribbons, breaking them up into thin strips, and sauté them until they brown.
As the onions begin to stick to the bottom of the pan, add a little water to unstick them as you stir and continue to sauté them and let them get progressively darker and sweeter. Repeat this process several times until they have reached the desired level of caramelization.
Roasting the vegetables
Meanwhile, roast the red peppers under your broiler and peel and de-seed them as explained in my post about making roasted red peppers.
Remove the caramelized onions from the pan, setting them aside, and add the pumpkin puree to the pan. If you are using a pre-made pumpkin purée or one that has been made by boiling, you can bring out the flavors by caramelizing the pumpkin a little bit, just as you did earlier with the onions. (You won't need to cook the pumpkin for nearly as long, though. You just want to bring out some of the sweetness of the pumpkin.)
Add in the roasted red peppers, half of the caramelized onions, the cumin, sage, salt, and pepper.
Add the water. (I used around 2 cups to make a thick soup, ready for decorating, but you can add more water to get a thinner consistency if desired.)
Warm the ingredients over medium heat for several minutes to get the ingredients to the desired temperature.
To obtain a creamy texture, blend the ingredients together for several minutes until smooth. You can either carefully use an immersion blender in the same pan or pour the mixture into a stand blender instead.
Serve immediately in bowls and garnish with the rest of the caramelized onions. You can also garnish with pumpkin seeds, baked proscuitto or serrano ham, or even a bit of greek yogurt as shown below!
For a thinner soup, add more water. You can also substitute the water for any broth of choice.
For a very smooth, creamy soup, blend for several minutes. A stand blender usually has more power and can make a smoother soup, but can be prone to explosions of hot liquids. When using a stand blender, make sure the soup isn't very hot and only fill the jar halfway each time or it can cause the top to pop off and make for a messy soup explosion. (Removing the center insert of the lid and covering it with a towel allows steam to escape safely and prevents the "explosion".)
This soup can be stored in the refrigerator for several days, but also freezes well if you make a larger batch and want to save some for another week.
You can decorate this soup with cream, greek yogurt, or sour cream.
Garnishes that pair well with this soup include prosciutto (or serrano ham), pumpkin seeds, and extra virgin olive oil.