Combine rice vinegar, granulated sugar, and oyster mushroom sauce in a small bowl; set aside
On high heat, add water and salt in a pot and bring the mixture to a rolling boil.
Stir in the spinach and blanch for 30 seconds.
Strain and place on an ice bath until it cools down.
Add in onion slices, tomatoes, and bell peppers:
Pour the dressing over and mix well
Serve immediately!