Crawfish Appetizer

1 pound crawfish tails 8 tablespoons salted butter 1 medium onion, chopped to equal 1 cup 3 tablespoons flour 1 tablespoon tomato paste 1/2 pint half and half to equal 1 cup 1/4 cup white wine (or more) 1 tablespoons chopped fresh parsley 1/2 teaspoon salt 1/2...