First prepare the tomato sauce. Place a skillet over medium-high heat. Add the olive oil, diced onion, chopped celery stick, chopped garlic cloves and chopped chili. Cook for about 5 minutes, stirring occasionally. Add the tomato puree (or tomato passata), chopped herbs and 150 ml (2/3 cup) of water. Season with salt and pepper and cook for about 30 minut on low heat (light simmer).
Clean the peppers and cut them in half
Cut the peppers lengthwise in half from top to bottom and discard the seeds. Fill the pepper halves with smoked mozzarella cheese and press halves back together. Repeat the process until you fill all the peppers.
Coat in eggs and flour
Lightly beat eggs in a small bowl and put flour in another bowl. Dip the stuffed pepper in eggs, letting excess drip off, then coat with flour and then coat with eggs again to form a second layer, letting excess drip off. Repeat the process until all the stuffed peppers are coated.
Deep fry the stuffed peppers
Heat the vegetable oil to 175°C / 345°F in a medium saucepan. Working in batches, fry stuffed peppers for about 3 - 4 minutes or until golden brown. Stir occasionally but be careful because the oil is very hot. Transfer to paper towels to drain.
Transfer the Smoked Mozzarella Stuffed Peppers to the skillet with tomato sauce. Cook for about 10 minutes, then sprinkle with chopped parsley and serve.