Passing a custom object from middleware to the client

[AstroSquared]

Here's some middleware in my entry-server.tsx:

export default createHandler(
   ({ forward }) => {
      return (event) => {
         const user = getUser(event)
         return forward(event)
      }
   },
   renderAsync((event) => <StartServer event={event} />)
)

How would I pass the user object from the middleware above onto the client, so that in my root.tsx file I can access the object?