8 ounces dried Korean sweet potato noodles
4 teaspoons toasted sesame oil
3 tablespoons soy sauce
1 tablespoon packed dark brown sugar
1 tablespoon vegetable oil
1 medium carrot, peeled and cut into 1-1/2-inch matchsticks
1/2 medium yellow onion, thinly sliced
3/4 teaspoon kosher salt, plus more as needed
1/4 teaspoon freshly ground black pepper, plus more as needed
4 ounces fresh shiitake mushrooms, stems trimmed and thinly sliced
2 medium garlic cloves, finely chopped
4 ounces baby spinach
1 medium scallion, thinly sliced (white and light green parts only)
1 1/2 teaspoons toasted sesame seeds