How do you Hire a Sr. Engineer that has much more experience than you?

Focus on reading the personality of the candidate for now. A solid personality almost invariably means a solid life-long learning attitude, which translates into skills that are forefront in industry.

You can have the best techologist with the worst personality, and he/she will do more damage than good.

It's a lot more difficult to find candidates with solid personalities than those with good tech skills.

As for vetting tech skills, you can't during the interview. Esp not in your position. But ask the candidate this:

"How do we know you're doing the right thing, technologically speaking?"

Ans: "Even if my managers don't know my tech stuff, I always install a workflow that conveniently and efficiently maintains 'full visibility'. Peer reviews will already spot my errors. In worst case, casual reviews of my recorded work months later can still call out my bluffs, even if my managers aren't entirely tech-savvy."

Really interesting question, Mike. Here's my two cents:

If you were tasked with hiring an engineer significantly more senior than you, how would you approach the interview?

I would approach this as an exploration of their experience rather than trying to measure skills or knowledge directly. Have a conversation about work they've found challenging, projects they've enjoyed, a problem that had them scratching their head recently and how they solved it, a piece of work they've found frustrating, what kind of work they try to avoid, how they've dealt with legacy code they ended up inheriting, how they've dealt with reviewing code that was problematic (and what was the problem), etc.

Discussing these kinds of things should help reveal what level the candidate is relative to you without needing to ask specific technical knowledge questions. Devs at any level will be able to answer all but the last two, but you'll find the experienced devs talk about much more complex and nuanced problems/topics. I'd also personally find an interview like this really fun fwiw.

Think of questions where the answer can be anywhere between printing "hello world" and landing a rocket on a barge in the middle of the ocean.

Or is it more important that you work well together and less about technical ability?

To some degree, yes - you should be able to establish if someone has more experience than you, but you probably can't objectively judge just how big that gap is outside of "they're where I want to be" and "I'm completely blown away".

If a decent amount of the interview asynchronously via some questionnaires over email, it will give you some more time to gather your thoughts and get outside feedback. This could take some of the pressure off the synchronous parts.

Come time to talk face-to-face, I’d say that if they can genuinely teach you something, they’ve shown a lot of technical and interpersonal skills, so maybe try to set them up with something to teach you.

Anyway, those are my first couple thoughts.