In a small bowl, mix the soy sauce, mirin, cornstarch, water, and sesame oil. Set aside.
Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced flank steak to the skillet, season with salt and pepper, and stir-fry for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the garlic and ginger, stirring for about 30 seconds until fragrant.
Add the asparagus and sugar snap peas to the skillet, cooking for 2-3 minutes until they are bright and tender-crisp.
Return the beef to the skillet and pour the sauce mixture over everything. Stir well to combine and cook for an additional 2 minutes until the sauce thickens.
Serve hot over cooked rice or noodles.