Slow Cooker Ginger Beef Udon Noodle Bowl

Name: Slow-Cooker Ginger Beef Udon Noodle Bowl
Ingredients: 1 1/2 cups (12 fl. oz./375 ml) beef stock; 1/3 cup (3 fl. oz./80 ml) soy sauce; 1/4 cup (2 fl. oz./60 ml) mirin; 1 yellow onion, finely chopped; 2 Tbs. firmly packed dark brown sugar; 2 Tbs. peeled and grated fresh ginger; 1 boneless beef chuck roast, about 3 lb. (1.5 kg), tied; Kosher salt and freshly ground pepper; 1 Tbs. canola oil; 1/2 lb. (250 g) shiitake mushrooms, stems discarded and caps sliced; 1 bunch green onions, including tender green parts, thinly sliced; 1 1/2 lb. (750 g) dried udon noodles; Shichimi togarashi for garnish (optional)