Heat oil in a large skillet or wok on high heat for 2 minutes.
When hot, add the onion and celery and stir-fry for 3 minutes.
Add the shrimp, sliced mushrooms, and minced garlic, and continue to stir-fry for 1 minute or until the shrimp turn pink.
Add the shredded spinach and peas and stir-fry for 30 more seconds.
After combining the cornstarch, water, and reduced-sodium soy sauce, add it to the shrimp mixture. Continue to cook, stirring occasionally over medium-high heat, until the mixture thickens. Serve immediately.