Prepare the potatoes: place whole Yukon Gold or baby potatoes in a medium pot, cover with cold water by 1" and season the water with 1 tsp kosher salt. Bring to a boil and simmer until just tender when pierced with a knife (about 12–15 minutes). Starting in cold water and cooking to just-tender helps the potato interiors cook evenly without falling apart when smashed.

Drain the potatoes and let them steam-dry in the colander for 2–3 minutes to remove surface moisture. Place on a rimmed baking sheet, gently press each potato with the bottom of a glass or potato masher to 'smash' to about 1/2" thickness. Doing this increases the surface area so you get more crispy edges when frying.

Season the smashed potatoes with 1/2 tsp kosher salt and a few grinds of black pepper. Heat 2 tbsp vegetable oil in a large nonstick or cast-iron skillet over medium-high heat until shimmering. Add potatoes in a single layer (work in batches if necessary) and cook without moving for 4–5 minutes to form a golden crust, then flip and crisp the other side 3–4 minutes. High direct heat creates Maillard browning for deep flavor and crunchy texture.

Transfer the crisped potatoes to a plate and keep warm. Wipe the skillet lightly with a paper towel, reserving any brown bits — those are flavor for the sauce.

While potatoes cook, slice the steak thinly across the grain into 1/8"–1/4" strips. Cutting against the grain shortens muscle fibers and makes the beef tender; very thin slices sear quickly and stay juicy.

Make the sauce: in a bowl whisk together 1/4 cup soy sauce, 1 tbsp brown sugar, 1 tbsp rice vinegar, 1 tbsp sesame oil and 1 tbsp cornstarch with 2 tbsp water until smooth. The cornstarch will thicken the sauce rapidly when it hits the hot pan; mixing it cold prevents lumps.

Heat 1 tbsp vegetable oil in a large wok or skillet over high heat until just smoking. Add half the beef in a single layer and sear, undisturbed, for 30–40 seconds until a deep brown crust forms, then toss/flip and cook another 20–30 seconds — you want a quick high-heat sear to caramelize the exterior while keeping the interior medium-rare to medium. Remove to a bowl and repeat with remaining beef. Working in batches avoids crowding, which would steam the meat instead of searing it.

Lower the heat to medium-high and add the minced garlic and ginger to the pan with a splash (about 1 tsp) of oil if the pan is dry. Stir for 20–30 seconds to release their aromatics — you only want to cook them briefly to avoid bitterness from overbrowning.

Add the carrots and broccoli first (they take longest). Stir-fry 2–3 minutes until they begin to brighten but remain crisp-tender; then add the red bell pepper and the cooked beef back to the pan. Building the stir-fry in stages ensures even cooking and keeps vegetables vivid and crisp.

Stir the sauce again (cornstarch tends to settle) and pour it into the wok. Toss everything quickly; the sauce should come to a gentle bubble and thicken in 30–60 seconds, coating the meat and vegetables. Quick thickening lets the sauce cling to ingredients without reducing into a syrup.

Finish with 1 tbsp butter and the green onions; toss to combine. The butter adds gloss and rounds the flavors while the green onions provide fresh aromatics and a mild bite.

Plate by arranging a bed of smashed potatoes, spoon the beef and vegetable stir-fry on top or alongside, and sprinkle toasted sesame seeds and additional green onion. Serve immediately. Serving right away preserves the potatoes’ crispness and the stir-fry’s hot texture.