Heat oil in a wok or large frying pan on high heat. Add garlic and stir-fry for 30 seconds. Add mushrooms and stir-fry for 30 seconds. Add snow peas, red pepper, onions, and baby corn and stir to combine.
Add soy sauce, water (if needed), sugar, sesame oil, and stir-fry for 2 minutes, until everything is shiny and hot-crisp.
Add cornstarch/water mixture, and stir-fry for 1 more minute, until the sauce has thickened somewhat.