Add water to cover both the vegetables and the shells which will be added later. Simmer for 30 minutes.
If using crab or lobster shells, use a rolling pin to break the shells into smaller pieces. Place shells on a large foil-lined baking sheet and put the smashed garlic cloves on top.
Roast the shells and garlic for 10 to 15 minutes, stirring halfway through.
After the vegetables have simmered for 30 minutes, put the roasted shells and garlic into the stock pot and add remaining ingredients. Add additional water, if necessary, to cover.
Simmer for 20 to 30 minutes, but no longer or your stock will start to become bitter and lose its flavor.
Strain the stock, and if desired, reduce over a low simmer until it is concentrated to your liking.