Drain off any excess grease from the pot (or dab it up with paper towels) and add 1 tablespoon olive oil. Return the skillet to MEDIUM heat and add the onion, celery, carrot, and bell pepper. Cook, stirring occasionally, until starting to get tender, about 5 to 6 minutes. Add the garlic and cook for an additional minute or two. Return the cooked sausage to the pan and add the broth, tomatoes, basil, parsley, Italian seasoning, salt, and pepper, and bring to a boil. Stir in the zucchini and orzo and return to a boil, then reduce the heat to MEDIUM-LOW and cook, covered, until the zucchini and orzo are tender, 12 to 15 minutes.