Earth Casserole

This is a delicious cold weather dish, but I'm posting it in July for all my fellow UK residents in commiseration over our autumnal weather!  [serves 6-8 people]

Saute in a tablespoon of olive oil:

• one leek, quartered lengthwise and sliced 
• two medium red onions, diced
• one bunch of green salad onions, chopped
• four garlic cloves, crushed
• one red bell pepper, diced 
• 1 TBS each of dried thyme and oregano

After five minutes, add a 690g jar of passata (thick pureed tomatoes).  Cook for ten minutes, then remove from heat and salt and pepper well. 

• One whole, medium-large celeriac: peel and chop into 1" cubes
• Five medium red potatoes, cleaned and cubed  
• Four medium sweet potatoes, peeled and chopped  

Lightly steam all veggies for about fifteen minutes.  Toss with olive oil, salt, and pepper in a large baking pan.  Stir in a pot of ricotta or goat's milk cheese.  Pour tomato sauce mixture over cheesy veggies, and top with freshly grated parmesan or cheddar cheese.  Bake 180C/350F/gas mark 4 for an hour.