Keep the bell peppers whole, making a horizontal slit while leaving the stems intact. Create pepper pockets by removing the seeds and membranes and set them aside.

Finely chop the chicken fillet, heat a non-stick skillet, and sauté the chicken in olive oil for 5-6 minutes. Add the onion and sauté for another 3 minutes. Add the tomatoes, herbs, salt, and pepper. Stir in the bulgur and simmer for 5 minutes. Add half of the grated mozzarella and the parsley to the filling.

Line a baking dish with parchment paper and place the bell peppers in it. Fill the peppers with the mixture and bake in a preheated oven at 180°C (356°F) for 30 minutes. In the last few minutes of baking, sprinkle the remaining mozzarella on top to melt.