Korean Japchae

Name: Korean Japchae
Ingredients: 10 ounces boneless rib eye steak, thinly sliced; 4 1/2 tablespoons soy sauce, divided; 2 1/2 tablespoons light brown sugar, divided; 3 1/2 tablespoons toasted sesame oil, divided; 2 garlic cloves, minced (about 2 teaspoons), divided; 10 ounces fresh baby spinach (about 10 cups); 1 1/4 teaspoons kosher salt, divided; 12 ounces uncooked Korean sweet potato noodles or mung bean noodles; 3 tablespoons canola oil, divided; 1 1/3 cups 3-inch-julienne-cut carrots; 2/3 cup 1/2-inch-sliced white and light green scallion bottoms; 1 cup 1-inch-sliced dark green scallion tops; 2 cups thinly sliced shiitake mushroom caps; 1 tablespoon roasted sesame seeds (optional)