With a serrated knife, score an X across the top of each chestnut. A wide X across the top is ideal, make sure to cut through the membrane layer below the shell too. Be careful with the knife, chestnuts can be slippery, place them on a tea towel when cutting to stop them escaping.
Once all chestnuts are scored with an X place them in a saucepan and cover with cold water.
Place saucepan on a high heat and remove chestnuts as soon as the water begins to rumble. Tip chestnuts into a colander to drain the hot water.
Transfer chestnuts to a cooking sheet pan, X cut sides up, and place in a preheated oven at 425 and roast for 20 mins.
Trim brussels sprouts and remove damaged leaves, rinse and pat dry, then cut them in half.
Place brussels sprouts into a large bowl and drizzle with 2 tbsp of olive oil and sprinkle with kosher salt, then transfer to a cooking sheet pan and place in oven.
Check how the chestnuts and brussels sprouts are doing a couple of times, you will notice the shell will open away from the chestnuts. Shake brussels sprouts around the cooking sheet to get color on all sides.
After 20 mins remove chestnuts and brussels sprouts from the oven.
When chestnuts are cool enough try one to make sure they're roasted properly, return to the oven if necessary.
Brussels sprouts should be cooked through with just a little bite to them. Set aside until needed.
Once the chestnuts are roasted tip into a tea towel to keep warm.
As soon as you are able to handle the chestnuts remove the shells and discard and set aside meat of the chestnuts until needed.
Cut bacon into thin strips and cook, then transfer to a paper towel lined plate. Set aside until needed.
In a saucepan heat butter with 1 tsp of olive oil on a high heat.
When butter is almost completely melted add onion and cook until flavor releases.
Add brussels sprouts and chestnuts and a bay leaf, then cover with 4 cups of chicken stock.
When the liquid begins to rumble remove from heat.
Add 2 cups of cold chicken stock to blender, add contents of saucepan, then add sour cream.
Blend on high until silky smooth, then return to saucepan and cook for 5-10 minutes to allow flavors to combine.
Add salt and pepper to taste.
Serve when ready and garnish with bacon and walnuts.