Turn the Instant Pot on to Saute setting. Melt reserved beef drippings, then add in the carrots, celery, onions, and garlic. Cook for 5 minutes. Then add in the diced roast beef and the broth.
Turn the Instant Pot off then select "soup" setting. Set for 15 minutes. Once done cooking, carefully do a quick release pressure. Add in the plum tomatoes, barley, and frozen green peas (add the sliced mushrooms if you want them in the soup). Place the lid back on the Instant Pot and set for 1 minute on Manual Setting. Once done do a quick pressure release.
Season with salt and pepper and ladle into bowls. Garnish with fresh chives and flat leaf parsley.
Calories: 80kcal | Carbohydrates: 14g | Protein: 5g | Fat: 1g | Saturated Fat: 1g | Cholesterol: 2mg | Sodium: 574mg | Potassium: 296mg | Fiber: 3g | Sugar: 3g | Vitamin A: 2885IU | Vitamin C: 10mg | Calcium: 35mg | Iron: 1mg