You’ll always find people argue about shepherd’s pie but in my view it should be made with lamb rather than beef (that’s cottage pie) and with very little, if any tomato - apart from maybe a dash of ketchup for sweetness.
Traditionally it was a frugal dish to finish up the Sunday roast, made with leftover lamb and gravy - and and any leftover vegetables that were going. I personally think it’s better made with fresh lamb mince but still like it kept simple and old-fashioned - a little onion, a little carrot, maybe a few peas, a touch of gravy or roast lamb pan juices and some nice creamy mash.
In terms of the ideal pairing it’s a toss-up between beer and a dry medium-bodied red wine:
* a classic English ale like Timothy Taylor’s Landlord is great with shepherd’s pie - possibly my top choice
* Stout or porter also works well, particularly if you’ve used it to make the gravy
* Red rioja is always good with lamb. I’d go for a reserva with this plain kind of dish rather than a younger, more vivid crianza. The same applies to other oak-aged Spanish reds such as Valdepeñas.
* Inexpensive red Bordeaux - what used to be called a ‘lunchtime claret’ - is a great pairing. Again I’d opt for a more traditional style rather than one with higher alcohol and a lot of ripe fruit. It won’t do any harm to a more expensive mature claret either. Simple food is best with special wines.
* A southern French red like the ever-versatile Côtes-du-Rhône Villages or a named village wine like Vacqueyras is always a good match - with shepherd’s or cottage pie.
PS Look, I'm not saying you need a totally different type of wine for cottage pie but the filling often tends to be more like a bolognese sauce - or more tomatoey, at any rate. So check out my recommendations for spag bol or, if you go for one of the above wines maybe choose a more fruity or full-bodied version.