Finely chop onion and garlic and saute in olive oil in a large frypan over a medium heat until tender. Increase heat and add mince, cook until well browned, tossing regularly so as not to burn. Transfer mince to a large saucepan and add salt and pepper, water, beef stock, tomato juice, mushrooms and diced pumpkin. Simmer for 30 minutes. Meanwhile peel and cook potatoes in a saucepan of boiling, salted water until tender. Drain well. Add butter and mash until light and fluffy.
Add finely chopped spinach to the mince mixture and cook for a further 10 minutes. Thicken mince with a little flour mixed with some water or cooled liquid from saucepan.
Preheat oven to 200 C
Line a large dish with pastry and pour over hot mince. Cover with mashed potato and bake until pastry is puffed and golden.