How to Make It
Preheat oven to 400°F. Place chicken and beef bones, carrots, onions, and garlic in a single layer on a large rimmed baking sheet. Bake in preheated oven until browned, about 1 hour.
Transfer bone mixture and pan juices to a 6-quart programmable electric pressure cooker. Add water, celery, tomatoes, peppercorns, thyme sprigs, lemon peel pieces, and bay leaf. Close and lock lid of cooker; turn pressure release valve to sealing position. Program cooker to cook on Manual on High Pressure for 1 hour. Turn pressure valve to the venting position to quickly release pressure (steam) from cooker until float valve drops. Carefully remove lid.
Pour broth through a fine wire-mesh strainer into a bowl; discard solids. Using a ladle, skim fat from top of broth; discard. (For a more clear broth, strain a second time through cheesecloth.) Stir in salt, if desired, and serve hot.