VENISON BOURGUIGNON PIE with Celeriac Mash

The last pie before summer

I have two superpowers:
1) turn everything into a pie
2) talk for hours about M. F. K. Fisher. 
For now, let's focus on the power of pies, which I inherited from my friend northerner Sue, who used to say, "Let's stick some mash on it and call it a pie". 
Well, Sue was a colleague of mine and one of those female chefs like Angela Hartnett who, when they say something, you do it... without questioning (matriarchal Italian style). I've followed her advice and have so far made the most amazing concoctions from leftovers and the magical mash, like this one... with VENISON. Oh my deer.
(I'm 42 years old and I'm allowed to dad jokes!).

Beef Bourguignon (or Beef Burgundy) is a French beef stew cooked in red wine (preferably from Burgundy, but who has time for that) and served with pearl onions and cup mushrooms.
It's very popular in England because it's the quintessential dish from Come dine with me, I do not know how many contestants cooked it.
We have all seen the show, there's no shame in it, a British family favourite (the show and the dish).
All the gay people I know have cooked it for me at a dinner party. You guys are obsessed with this dish.
Well, so am I. It's rich in flavours, has its own flamboyance and leaves guests guessing how to pronounce it correctly (I've been there with my name too, and once and for all: NO YOU CAN’T CALL ME EMILIO).

Fun fact there is something similar that my Japanese flat mate cooked for me when we were young and broke chefs just landed in Albion. It's called “Hayashi” and is a Western-style dish made with braised beef, onions and mushrooms. It's made with a demi-glace sauce (red wine and tomato sauce) that binds everything together, but in my opinion it's more like a Japanese curry. In Japan, there are many sachets to prepare this dish and it is usually served with rice.

In 1868, the trading market in Japan's harbour cities was very active. The foreign influence introduced the practice of eating beef and many Western dishes that developed into a proper sub-genre of cuisine if you think of tonkatsu, spaghetti Neapolitan, Chaliapin steak. Hayashi has been a very popular dish ever since.