In a small bowl, whisk soy sauce, water, sesame oil, rice vinegar, maple syrup, cornstarch, and red pepper flakes. Set aside.
Wash and cut broccoli into florets, or thaw if frozen. Slice mushrooms, bell pepper, mince garlic, and grate ginger.
Heat 2 tbsp vegetable oil in a skillet or wok over medium-high heat. Add broccoli, mushrooms, and bell pepper, and cook for 5-7 minutes, stirring often.
Add garlic and ginger to the skillet. Cook for 1 minute, stirring constantly.
Stir the sauce again, then pour into the skillet. Cook for 2-3 minutes until the sauce thickens.
Stir in spinach or kale and cook for 1-2 minutes until wilted. Season with salt and pepper.
Sprinkle with green onions. Serve hot over rice, noodles, or on its own.