Crush the shells a bit with a hammer or meat mallet (or skillet). Brush with a little olive oil and roast in a 400° F oven until they are bright red - about half an hour. They will smell awful, so make sure your exhaust fan is on or that you can open a window.
Brush some tomato paste on the shells during the last fifteen minutes of roasting.
Put the roasted shells in a stock pot along with the rest of your ingredients. Cover with cold water, and slowly bring the temperature up to a simmer. Adjust heat to maintain a slow simmer. Simmer for about 1½ to 2 hours. Strain and chill.
To save storage space, you can reduce the stock by half by gently boiling on the stove. Either way, store in the refrigerator for a week or in the freezer for 3 months.