Boil the water and the onion in a medium pot

While waiting, chop the spinach stem into desired size

When the water boil, put the corn kernel into the pot and let the water boils again (this step can be skipped if you dont like corn)

When the water boils again, put the spinach stem into the pot and let it boils

Finally, add the spinach leaves into the pot, pour the salt and heat the pot for another few minutes or until the texture of all ingredients meet your desire

You can also cook chinese okra, which resembles zucchini, in the same way