Begin by thinly slicing the flank, skirt, or sirloin steak against the grain to ensure tenderness. Lightly season the beef with salt and pepper, then toss with a bit of soy sauce and cornstarch for added flavor and moisture retention. Let the beef sit while you prepare the other ingredients.

In a mixing bowl, whisk together low-sodium soy sauce, oyster sauce, sugar, freshly cracked black pepper, sesame oil, and the cornstarch slurry. Set the sauce aside.

Heat 1 tablespoon of vegetable or avocado oil in a large skillet or wok over medium-high heat. Add the prepared beef to the pan in a single layer. Sear for about 2-3 minutes, stirring occasionally until browned and cooked through but still tender. Remove and set aside.

In the same skillet, add the thinly sliced onion and minced garlic. Sauté for 1-2 minutes until the onion becomes translucent and garlic is fragrant. Avoid browning.

Add the cooked udon noodles directly to the skillet, followed by the sauce. Toss to coat and heat through for about 2-3 minutes.

Return the seared beef to the skillet, mixing thoroughly with the noodles and sauce. Continue to stir for another minute until heated and fully incorporated.