Preheat oven to 200°C / 200° Fan / Gas 6.
Meat & lentil baseHeat the tablespoon oil in a large pan over a medium heat. Add the onion and celery and cook gently for 8-10 minutes until softened but not coloured. Stir in the garlic and cook for 1 minute more. Increase the heat slightly, add the minced beef and cook for 4-5 minutes, breaking it up with a spoon, until browned.
Stir in the tomato purée and thyme and cook for 2 minutes. Add the crumbled stock cube, lentils, chopped tomatoes, Worcestershire sauce and diced carrot.
Bring to the boil, then reduce the heat and simmer gently for 15 minutes until the sauce has thickened slightly.
Stir in the peas and cook for a further 3-4 minutes. Season with freshly ground black pepper.
Prepare the mashCook the potatoes in a large pan of boiling water for 15-20 minutes until tender. Drain well and return to the pan. Place the cannellini beans and milk in another small saucepan and warm gently for 3-4 minutes. Take off the heat and blend until smooth using a handheld blender or small food processor.
Add the bean mixture and olive oil to the potatoes and mash until smooth, adding a splash more milk if needed. Season with plenty of freshly ground black pepper.
Assemble & bake.Transfer the beef and lentil mixture to a large ovenproof dish and spoon over the mash topping, smoothing lightly with the back of a spoon.Bake for 25-30 minutes until golden and bubbling around the edges. Serve on its own or with seasonal vegetables.