Shrimp Chowder

4 large onions, peeled and sliced
1/4 cup butter or margarine
1 cup boiling water
6 medium potatoes, pared and cubed
1 tablespoon salt
1/2 teaspoon pepper
6 cups milk
2 cups grated sharp Cheddar cheese
2 pounds raw shrimp, shelled and deveined
3 tablespoons snipped parsley

In Dutch oven saut