Blend water into cornstarch; stir in soy sauce and salt.
Snap off and discard the woody bases from asparagus.
Bias-slice asparagus crosswise into 1 1/2-inch lengths.
Preheat a wok or large skillet over high heat; add cooking oil.
Stir-fry asparagus and green onions in hot oil for 4 minutes.
Add mushrooms; stir-fry 1 minute more.
Stir soy mixture; stir into vegetables.
Cook and stir till thickened and bubbly.
Add tomatoes and heat through.