YIELD: 2 quarts | PREP TIME: 10 minutes STOVETOP: about 1 hour | SLOW COOKER: 6 to 8 hours | PRESSURE COOKER: 20 minutes 2 pounds fish bones and heads (preferably from lean white fish such as halibut, cod, or flounder) 1 tablespoon apple cider vinegar 1 large onion,...
