1. Shell the shrimp and devein them by making a shallow incision down the back with a small, sharp knife, then lifting out the black or white intestinal vein with the tip of the knife. Set the shells aside.
2. In a 5-quart, flameproof casserole, heat the oil over moderate heat until a light haze forms above it. Add the onions, shrimp shells, fish trimmings and garlic, lower the heat to moderate, and cook, stirring constantly, for 6 minutes, or until the onions are transparent but not brown and the shells are pink.
3. Stir in the tomato, chili, oregano, salt and pepper, and cook for about 3 minutes, stirring frequently. Add the water and diced potatoes, and mix thoroughly.
4. Then cover the casserole, reduce the heat to low, and simmer for 30 minutes, or until the potatoes are soft. Pour the entire contents of the casserole into a sieve set in a large bowl, and force the ingredients through with the back of a large spoon.
5. Discard the shells and any pulp remaining in the sieve. Return the pureed mixture to the casserole, bring it to a boil, and add the potato halves and the rice.
6. Cover, reduce the heat to low, and simmer for 25 minutes, or until the potato halves are almost tender, then add the peas and corn, and simmer, covered, for 5 minutes longer.
7. Drop in the shrimp and cook for 5 minutes, or until they are firm to the touch and pink. Meanwhile, beat the eggs in a small bowl until they are well blended, then slowly pour them into the stew, stirring constantly, until they coagulate into shreds.
8. Pour in the milk and simmer only long enough to heat it through. Taste for seasoning. Sprinkle the stew with parsley and serve at once.
