Vegan Spicy Udon Noodle Soup

Name: Vegan Spicy Udon Noodle Soup
Ingredients: 1 tablespoon (15 ml) canola oil; 1 small yellow onion, diced; 2 ribs celery, sliced; 1 large carrot, julienned or diced; 3 large cloves garlic, minced; 1 cup (60 g) bunashimeji or enoki mushrooms (long, skinny mushrooms) (optional); 9 cups (2.12 L) water; 3 vegetable bouillon cubes; 2½ cups (190 g) shredded napa or green cabbage; 1 (12-ounce, 340 g) package dry udon noodles; 1 tablespoon (15 g) Sriracha sauce; Ground black pepper, to taste; 2 green onions, sliced, for garnish (optional); ½ sheet sushi nori (toasted seaweed), finely sliced, for garnish (optional)