Step 1Heat the oil in a medium stockpot over low heat. Add the onion, garlic, carrot, tomatoes and parsley stems and cook, covered, until soft, about 10 minutes. Stir from time to time to keep from sticking.
Step 2Add the shrimp and mussels and stir to coat with flavorings. Add the water, salt and pepper. Bring to a simmer over medium-high heat; as soon as gentle bubbles form on the surface, reduce the heat to low. Cook 35 minutes.
Step 3Drain the stock through a strainer, then cover and refrigerate the liquid until cool. Reserve about 1/4 pound of the cooked shrimp and refrigerate. Discard the remaining solids.
Step 4To clarify the consomme, remove the broth from the refrigerator and discard the fat on the surface. Heat the broth in a large saucepan or small stockpot over medium-low heat. While the broth is warming, shell the reserved cooked shrimp and put the meat in the food processor. Add the egg whites and puree until you have a light pink foam, about 30 seconds.
Step 5Whisk the shrimp mixture into the broth and continue to heat, whisking steadily until it boils, then reduce the heat to a simmer. Keep whisking slowly but constantly. After about 30 minutes, small bits of egg white will begin to accumulate into larger pieces, about the size and shape of snowflakes. Stop whisking and let the mixture continue to heat. The egg white bits will eventually collect into a moist "cap" on top of the broth. Poke a hole in the center with a spoon to allow the broth to bubble without overflowing. The broth should be in constant gentle motion, not boiling hard. Cook 1 hour without stirring.
Step 6Using a slotted spoon or Chinese wire skimmer, very gently lift the cap from the broth and discard. Ladle the broth into a fine-mesh chinois; otherwise, line a strainer or colander with moistened paper coffee filters. Pour the broth gently, ladling the liquid against the sides of the strainer and moving from spot to spot to avoid pushing any egg white through into the consomme. The finished consomme should be completely clear. If it is still cloudy, repeat the clarification process. If the flavor lacks intensity, reduce the consomme over high heat 5 to 10 minutes.