Preheat the oven to 180 degrees C
Place the potatoes in a large saucepan and cover with plenty of boiling water
Bring to the boil and simmer until soft
Meanwhile, heat the oil in a large saucepan
Add the onion and cook for a few minutes
Add the carrots, celery and garlic and stir
Continue cooking on a gentle heat for a few minutes
Add the tomatoes, tomato puree and stock
Add the lentils
Add the mixed herbs, cumin, paprika and chilli
Bring to the boil and turn down to a gentle simmer for about 20 minutes
Add the frozen peas and green vegetable and stir
Cook for a few more minutes
When the potatoes are cooked pour on the milk and add the butter
Mash with a masher until smooth
Place the lentil mixture in the bottom of a casserole
Place the mashed potato on top
Sprinkle with the cheddar cheese
Bake in oven at 180 degrees C for about 40 minutes