Tuscan Vegetable Tortellini Soup

1/2 cup olive oil
2 cups yellow squash, quartered, sliced
1/3 cup carrots, sliced
2 1/4 cup yellow onion, diced
1 1/2 cup celery, diced
1/4 cup garlic, minced
1 (50-oz.) can vegetable foundation
150 oz. water
2 tbsp. Italian seasoning
1 tsp. ground black pepper
3/4 cup frozen spinach, chopped
2 lb. frozen cheese tortellini
Shaved parmesan cheese, chopped basil, as needed