In a small bowl, whisk together cornstarch, vegetable broth, tamari, and sesame oil until smooth. Set aside.
Heat oil or water in a large wok or skillet over medium-high heat.
Add sliced onion and sauté until translucent, about 3-4 minutes.
Add minced garlic, grated ginger, and broccoli florets. Stir to combine.
Pour 1/4 cup water or broth into the pan, cover, and steam broccoli for 5 minutes until bright green and tender-crisp.
Add roasted cashews, any optional vegetables, and stir to combine. Cook for 2-3 minutes.
Pour the prepared sauce mixture over the vegetables and stir continuously until sauce thickens and coats everything evenly, about 1-2 minutes.
Serve immediately over cooked rice or quinoa.