Preheat the oven to 400°F (200°C).
Chop onion, bell pepper and celery finely and fry in butter in a large frying pan until slightly soft.
Season with salt and pepper.
Mix tuna, mayonnaise, parmesan cheese and chili flakes in a greased baking dish.
Add fried vegetables and stir.
Bake for 15-20 minutes or until golden brown.
Serve with baby spinach and olive oil.