If using the liver, thaw in the microwave on high for 2 minutes to soften, then cut into chunks. Place offal into stainless steel Instant Pot® insert, and top with frozen soup bones. * Add optional vegetables at this step.
Add 2 cups of water, secure lid and pressure cook on high for 2 hours. Allow pressure to release naturally, about 30 minutes or so.
Release remaining pressure and, using a slotted spoon, remove bones and meat from insert and spread out on sheet pan to cool.
Strain and measure remaining liquid, including fat, and add water if necessary to measure at least 4 cups. More is fine as the rice will absorb it all as it is stored.
Stir in 2 cups of rice and use rice setting or low pressure for 12 minutes. Quick release, laying a paper towel over the steam release to catch sputters.
While rice is cooking, remove all meat from the bones and discard the bones. If using the heads, all soft meat and connective tissue should be used, including eyes, tongue, cheeks, and brain. Chop the meat, offal, and optional vegetables into fine mince.
Combine cooked rice with the meat; pack into containers and refrigerate or freeze.