@User you can do:

ts
function myCustomMiddleware(verifySessionOptions?: Session.VerifySessionOptions) {
    return async (req: express.Request, res: express.Response, next: express.NextFunction) => {
        try {
            let session = await Session.getSession(req, verifySessionOptions);
            if (session === undefined) {
                if (verifySessionOptions?.sessionRequired === false) {
                    return next();
                } else {
                    throw Error("Should never come here");
                }
            }

            let userId = session.getUserId();
            // TODO: fetch info from mongo and attach to req.user = ....
            next();
        } catch (err) {
            next(err);
        }
    }
}

app.get("/some-api", myCustomMiddleware(), (req, res) => {
    // TODO:...
})

note that the above requires that you have added the supertokens errorHandler to the your express app (should be done if you follow the backend quick setup correctly).