Error formatting with trpcExpress.createExpressMiddleware

I'm trying to figure out how to do error formatting but it's unclear how to do it in the context of trpcExpress.createExpressMiddleware - The documentation use trpcInit.context().create({ errorFormatter(opts) { ... }) which I'm not using as I'm following the React Query example from the docs. Is there a concrete example I can refer to where should I plug the errorFormatter(opts)?