You will find chow mein noodles at a Chinese grocery stores or in the Asian food section of your market. Cook noodles per the direction on the package. Drain, rinse, and set in a colander to dry.
While the noodles are cooking, make the sauce by mixing al of the sauce ingredients into a small bowl and mixing to combine. Set aside.
Place oil in a large nonstick skillet and saute yellow onions for 5 minutes, stirring occasionally so that the edges get browned.
Add the green onions and cook for an additional 2 minutes.
Add in the cooked and drained noodles and the sauce, gently stir and cook for 5 minutes, stirring occasionally.
Garnish with toasted sesame seeds.

1 6-ounce package chow mein noodles
2 teaspoon high heat oil (grapeseed, sunflower, or peanut oil)
1 yellow onion, cut in half and then sliced
3 stalks scallion (green onion), thinly sliced

Sauce
1 teaspoon sesame oil
3 tablespoon soy sauce
1 teaspoon rice vinegar
1/2 teaspoon sugar
1/4 teaspoon white pepper