Preheat the oven to 180C/350F. In a large pan, which could go in the oven, brown the diced pancetta until the fat rendered, then remove to a separate plate. To the same pan ad the olive oil and seasoned with salt and pepper chicken breasts, brown until golden but not cooked all the way through. Remove to the same plate as pancetta. Now add the sliced bell peppers and sauté them for about 5 minutes while stirring, remove from the pan.
Reduce the heat to low and add sliced garlic to the pan, sauté for 20-30 seconds while stirring, do not allow it to brown, now pour in the tomato passata or crushed tomatoes, add the dried basil and oregano, bring to a simmer and salt to taste.
If you used an oven proof pan earlier bring the cooked pancetta, browned chicken breasts and sautéed bell peppers to the pan, scoop the sauce on top of the chicken breasts. If not, use a separate casserole, grease the bottom and add the chicken, pancetta and the bell peppers, now pour the sauce over them.
Scatter the grated mozzarella all over the chicken breasts and cook covered with a lid or aluminium foil in the preheated oven for 20 minutes, then uncover and cook for 5-7 minutes longer.