Spiralize the zucchini, leaving the skin on and stopping before the soft seeded center. Pat the noodles dry if they're visibly wet.
Whisk together soy sauce or tamari, rice vinegar, sesame oil, cornstarch, and red pepper flakes.
Heat a wide skillet over medium-high/high heat. Add oil, then cook bell pepper and mushrooms until just tender.
Add garlic and ginger and cook briefly until fragrant.
Add optional cooked protein, if using, and heat through.
Add zucchini noodles and toss gently over high heat for 60-90 seconds.
Pour in the sauce and toss for another 20-30 seconds, just until the noodles are glossy and coated.
Remove from heat immediately. Top with scallions and sesame seeds and serve.