Place 1 lb. sirloin steak on a cutting board and pat dry with paper towels. Slice meat crosswise as thinly as possible.

Transfer steak to a medium bowl and add 1 Tbsp. cornstarch, 1 Tbsp. soy sauce, and 1 Tbsp. vegetable oil. Toss until meat is evenly coated.

Prep the rest of your ingredients: Trim ends of 8 oz. snap peas and remove any strings; transfer to another medium bowl. Snap woody ends off of 1 bunch asparagus and discard. Cut asparagus crosswise into 1"pieces; transfer to bowl with snap peas. Trim both ends of 6 scallions and set aside 2 scallions for serving. Cut 4 remaining scallions crosswise into 1" pieces and add to bowl with asparagus and snap peas. Scrub 2" piece ginger under running water, then slice crosswise as thinly as possible.

Add the ginger to the bowl with the other vegetables.

Combine 3 Tbsp. Mirin, ¼ cup oyster sauce, 2 Tbsp. rice vinegar, and remaining 1 Tbsp. soy sauce in a glass measuring cup and incorporate. by stirring with a spoon.

Heat 2 Tbsp. vegetable oil in a large stainless steel skillet, over medium-high. When oil shimmers across surface of skillet, add vegetable mixture. Cook, shaking skillet often, just until asparagus are tender but still retain a hint of crunch, about 3 minutes. Return vegetables to original bowl.

Heat remaining 1 Tbsp. vegetable oil in skillet over medium-high. Add steak, arranging slices in a single layer. Cook, undisturbed, until juices start to pool on surface of meat and underside is browned, about 3 minutes. After 1 minute add minced garlic. Using a thin metal spatula, scrape bottom of skillet and loosen meat.

Immediately add cooked vegetables and sauce to skillet and cook, tossing constantly, until meat is fully cooked and sauce is thick and bubbling, 1–2 minutes more.

Remove from heat and let cool for a minute or two. Season stir-fry lightly with salt. Divide 2 cups cooked rice among plates. Spoon stir-fry over. Thinly slice reserved 2 scallions and scatter all over.