Sauté shrimp and crab shells with butter.
In a pot, add the shrimp shells and crab shells and sauté with butter over medium heat for 5-10 minutes or until shells have begun to brown and become aromatic.
Add all ingredients, bring to boil.
Add water, half the onion, a couple sprigs of thyme, dried bay leaves, and celery into the pot with the shells and bring to a boil. Let the mixture boil for 2 minutes or so.
Turn down to low heat and cover for 1 hr.
Turn the heat down to low, cover the pot, and let simmer untouched for 1 hour. Salt to taste. Remove from heat and store in mason jars. Congrats! You just made shrimp and crab stock.