2 cups shrimp shells
1/2 a red onion, cut in chunks
1 stalk celery, cut in chunks
1/4 cup Rao's Roasted Eggplant sauce or other low carb pasta sauce
3 tsp minced garlic
1 sprig italian flat leaf parsley
1 sprig thyme
1 sprig oregano
2 bay leaves
2 Tbs whole black pepper corns
8 cups water
Add all ingredients to the pot of your pressure cooker. Set to soup/stew for one hour. Strain through a strainer and discard everything but the stock.
*Optional: set for an additional hour for more flavor.
This is the pressure cooker I use: