Homemade Shrimp Stock

Preheat oven to 450 degrees. (See Notes)
Spread shrimp shells and heads in a single layer on a lined baking sheet. (Use two baking sheets if necessary).
Pour melted margarine over the shells; mix the margarine on all the shells.
Place baking sheets in preheated oven and let roast for about 20 minutes.

Add a little vegetable oil to a Dutch oven over medium heat; add the cut vegetables (yellow onions, carrot, and garlic), cut side down, to the stockpot; saute until slightly seared, about 8 minutes.
Add the roasted (with any liquid generated during roasting) or raw shells to the stockpot; stir and saute until the shells turn pinkish-red, about 5 minutes.
Add the celery (with leaves), Bouquets Garni, and the garni bag to the Dutch oven.
Add the cold water to about 1 inch above the shells.
Increase the heat to high and bring the liquid to a slight boil, then immediately reduce the heat to maintain a slight simmer. (See Notes)
Maintain a very low simmer for about 1 hour. Do not stir.
From time to time, skim off any impurities that rise to the surface. Do not stir the impurities back into the stock.
Turn off the heat and allow the stock to cool so it will be easier to handle and the flavors continue to blend.
Place a strainer or colander over a large bowl; using a slotted spoon, remove the solids and drain until all the solids have been removed.
Discard all the solids and return the drained stock back into the stockpot.
Place a fine mesh strainer, and pour the liquid through to remove any remaining solids. (See Notes)
The stock can be used immediately or stored in the refrigerator for a couple of days. If the stock will not be used within a couple of days, place the strained stock in freezer containers, label the containers, and freeze for up to six months.