I do several different versions. The original was in one of those Dairy cookbooks, roughly (for 2 but obviously just a guide):
Chop an onion, 2 celery sticks, and 2 carrots onto biggish dice. Cut a leek into short chunks, and plenty of mushrooms into halves/quarters.
Fry the onion, carrot, celery for a few mins, then add leek. Cook 5 mins with lid on, then add a tablespoon of paprika (not hot or smoked), salt & pepper. Stir in well, then add a sprinkle of flour, a dollop of tomato puree and a tin of tomatoes.
Cook with lid for about 15-20 mins, add mushrooms, then put in ovenproof dish.
Meanwhile cook potatoes, mash and mix in grated cheese.
Top veg with mash, sprinkle more grated cheese on top and bake about 20 mins 180c.
All very 'Ish'
Veg are inter-changeable. I add broccoli if I have it, or cauli, or a tin of any kind of beans if I'm short of veg. And red pepper if I have a piece hanging around, or a courgette.
For larger quantities, stick with one tin of tomatoes and add some veg stock.
Then I do an actual veggie shepherds pie. For that I cook green lentils. While they're cooking chop onion, carrot, celery finely (in mini-chopper), the idea is it all is mince-sized but sometimes I leave the carrot a bit bigger as that's how I do shepherds pie.
Fry the veg, add lentils once cooked. Add finely chopped mushrooms (really meaty taste from these). Add mixed herbs, salt & pepper and stock. I often use a beef oxo, because we're not veggie and it tastes nice, Worcester sauce too. Otherwise veg stock.
Mash on top as usual, plain or mixed with swede
