Boil the potatoes until tender. Peel and mash until smooth.
Add butter, grated kashar cheese, and salt. Mix well and set aside.
Heat oil in a skillet and sauté the chopped onion until soft.
Add diced chicken and cook until lightly browned.
Stir in mushrooms, green peppers, red pepper, tomatoes, tomato paste, tomato sauce, and seasonings. Cook for 8–10 minutes.
For the béchamel sauce, melt butter in a saucepan. Add flour and cook for 1 minute.
Gradually whisk in milk and cook until thickened. Season with salt and black pepper.
Spread mashed potatoes evenly in a baking dish.
Top with the chicken sauté mixture.
Pour the béchamel sauce over the chicken layer.
Sprinkle generously with grated kashar cheese.
Bake at 180°C (356°F) for 20–25 minutes or until golden and bubbly.
Let rest for 5 minutes before serving.