Heat up your pressure cooker: press Sauté -> click on the Adjust button -> select More to get the Sauté More function, which means that the food will be sautéed over medium-high heat. Wait for the Instant Pot indicator to read HOT.
Add the oil to the hot Instant Pot, add the diced onions and minced garlic, and sauté for 2-3 minutes, stirring a few times.
Add the carrots (plus celery if using) and cook for 1 minutes. Add the tomato paste and cook for another minute.
Add the rest of the ingredients to the Instant Pot, except those listed in the "Garnish" section. Stir.
Using manual function, set to high pressure for 9 minutes .
After 9 minutes, use quick release. Stir.
Serve topped with scallions, parsley and mozzarella cheese. Serve with crusty, warm bread on the side or grilled cheese.