Preheat the oven to 180°C.
Fry carrots and celery in 1 tablespoon of oil for 5 minutes until browned.
Add optional chicken bones and fry for 3 minutes.
Add garlic bulb, tomatoes, thyme, and parsley stalks.
Simmer 1.2 litres of water for 25 minutes, strain, and reduce liquid by one-third.
Fry onions in 1 tablespoon of oil until browned and stir in coriander and seasoning.
Fry peppers in 1 tablespoon of oil until softened and season.
Layer half the onions, half the peppers, and half the potato slices in a pie dish.
Repeat layers and pour the warm stock over the peppers.
Top with remaining potatoes and brush with melted butter and 1 tablespoon of oil.
Bake for 30 minutes until golden and bubbling.
Sauté sliced garlic in butter and oil for 2 minutes.
Fry mushrooms until colored and stir in coriander.
Serve the casserole with the garlic mushrooms on the side.