4 medium white onions, chopped
6 white poblano chile peppers, seeded and chopped
12 ounces shrimp, peeled and deveined
2 tablespoons Greek yogurt
1 tablespoon vinegar
1/3 cup boiling water
1 teaspoon dry mustard
salt and pepper to taste
1/4 cup chopped fresh parsley
Place the onions, peppers, shrimp and Greek yogurt into a small bowl and cool until liquid has evaporated; drain and reserve.
In a large saucepan, add boiling water, dry mustard, salt and pepper. Stir and gently simmer over low heat for 15 minutes.
Stir the vinegar and remaining liquid into the boiling water to the simmer gradually; remove from heat. Mix with the parsley and poblano chile sauce and pour mixture over the fish and vegetables in the slow cooker.
Cover and cook on low for 5 to 6 hours, using a 1-quart sealable canning dish.