Check if a stubbed getter function has been called with sinon spy - unit-testing

Question

I am using firebase admin and I am trying to write some unit tests for my code.
Since admin is injected in my function I figured I could mock a very simple object like this:
admin = {
get auth () {
return {
updateUser: () => {
return true;
},
createUser: () => {
return true;
},
getUser: () => {
throw Error('no user');
}
};
}
};
Then in a particular test I can stub the functions. Here is what I have done so far:
// stubbed functions
sinon.stub(admin, 'auth').get(() => () => ({
updateUser: () => ({ called: true }),
getUser: () => (userRecord),
createUser: () => ({ called: false })
}));
and those are working fine (I can see with my logs).
However in my test I would also want to check if createUser has been called at all.
I thought I could set up a spy on the createUser function, but so far I can't really get it to work.
Here is what I have been trying (with a bunch of variation always failing):
it.only('should update a user', async () => {
const userRecord = mockData
sinon.stub(admin, 'auth').get(() => () => ({
updateUser: () => ({ called: true }),
getUser: () => (userRecord),
createUser: () => ({ called: false })
}));
const spy = sinon.spy(admin, 'auth', ['get']); // this is not working
const user = await upsertUser(data, firestore, admin);
expect(user).toEqual(data.userDataForAuth); // this one is ok
sinon.assert.calledOnce(spy.get); // this throws an error
});
the bit of code I am trying to test (which is the upsert function is this:
// in my test exisiting user is not null (the stub `getUser` is returning a object
if (existingUser != null) {
try {
await admin.auth().updateUser(uid, userDataForAuth);
return userDataForAuth;
} catch (error) {
console.log('error', error);
throw Error('error updating user');
}
}
I am not even sure this is the best approach, happy to change it if there is a better one!