Boil a large pot of water. Peel and slice the onion into quarters and peel and finely chop the garlic. Deseed and slice the red bell pepper. Seperate the green and white parts from the bok choy. Cut the white part into cubes and just roughly chop the green part. Keep them separated because we´re going to add them at different times. Slice the spring onion.
You want to start cooking the spaghetti/noodles first so that you can do the rest while the spaghetti/noodles is boiling! Cook the spaghetti/noodles according to package instructions. Once cooked, drain and make sure there´s no water left or else the lo mein will get a bit soggy and saucy.
While the spaghetti/noodles is cooking, heat up a little oil to a wok or a pan. Start by sautéeing the onion and garlic. Once the onion is soft and translucent, add in the red bell pepper and the white part of the bok choy. Sauté for about 2 minutes.
Add in the sambal or chili paste and sauté for about 1 minute. Then add in the cooked spaghetti/noodles and stir to combine. Add in the soy sauce and the sweet soy sauce. I honestly just eyeball those two ingredients. Add in a little at a time, keep stirring and tasting until you think it's perfect!
At the last few minutes, add in the green part of the bok choy and the spring onion. Stir a final time until the green part of the bok choy is wilted.