Bring a large pot of water to a boil and cook 8 ounces Korean sweet potato starch noodles (dangmyeon) according to package instructions until al dente. Drain in a colander, rinse under cold water until cool, drain well, and set aside.
While the noodles cook or immediately after, heat 1 tablespoon sesame oil in a large skillet over medium heat until shimmering.
Add 2 teaspoons minced garlic, ½ cup thinly sliced onions, ½ cup thinly sliced carrots, and ½ cup thinly sliced bell peppers to the skillet. Sauté, stirring occasionally, until the vegetables are just tender, about 4–6 minutes.
In a small bowl, whisk together ¼ cup low-sodium soy sauce, 2 tablespoons gochujang, 1 tablespoon rice vinegar, and 1 teaspoon sugar until smooth. Pour the sauce over the sautéed vegetables and cook, stirring, for 1–2 minutes.
Add the drained noodles and ½ cup fresh spinach leaves to the skillet. Toss or use tongs to combine and cook just until the noodles are heated through and the spinach is wilted, about 1–2 minutes.
Remove from heat, divide onto plates or a serving dish, and sprinkle 1 tablespoon toasted sesame seeds over the top before serving.