Microwave broth in a microwave-safe bowl at HIGH 2 to 3 minutes or until simmering. Place ancho chile peppers in a blender. Pour hot broth over peppers; let stand 10 minutes. Holding lid down with a towel, process peppers and broth until smooth. Sprinkle flour, cumin, and salt over London broil. Place beef in a 4-qt. slow cooker. Add chopped onions, minced garlic, oregano, beer, 1 cup water, and pepper mixture. Cover and cook on HIGH 3 to 4 hours (or on LOW 5 to 6 hours) or until beef is tender. Top with halved jalapeño peppers, sour cream, and refrigerated salsa. Serve with saltine crackers.
Note: We tested with Los Chileros de Nuevo Mexico whole ancho chiles.