In a large pot add your salmon scraps (yes heads and spines and stuff), onion, and fill with water. Bring to a boil and then down to a simmer for about 2 hour. As it is boiling, check it ever 20 minutes or so to scrape off the foam that rises to the top.
After 2 hours the soup should be an orange-brown color. strain the contents of the pot. Save all the liquid in one bowl, and dump the scraps on a large cutting board or plate. Pick at the scraps by hand to remove any last good bits of meat.
Place the liquid and the salvaged meat into another clean pot (I used my trusty Emile Henry dutch oven) and slowly bring up to heat. While this is going you can fire roast your red peppers for a few minutes to get them a nice char.
Add the bell pepper to the soup along with the chives, then blend everything together with a stick blender. Don't worry if it doesn't get completely smooth, just blend as best you can. I put the immersion blender directly into the pot and just let it do its thing for a few minutes. Then add the heavy cream and stir it up. Season to taste.
Pour the contents of the pot into another bowl through a fine mesh sieve so only a cloudy-clear soup makes it into the bowl. You will be left with a fish paste in the sieve which is where a lot of the flavor is. You should reincorporate at least a tablespoon of this paste into the soup if you want. It will make it a bit thicker, but also gives a more robust color and taste.
Place the soup in a bowl, drizzle with dill oil, and garnish with some pea sprouts and chopped green onion.