Begin by slicing tender flank steak or sirloin against the grain into thin strips. In a mixing bowl, coat the beef strips in cornstarch, seasoning them with salt and pepper.

In a large skillet or wok, pour in about 2 tablespoons of vegetable oil and heat over medium-high heat until shimmering.

Carefully add the coated beef strips to the hot skillet and sear for about 2-3 minutes until browned and crispy. Remove and set aside.

In the same skillet, add more oil if needed, then add chopped bell peppers, onion, and minced garlic. Stir-fry 2-3 minutes until crisp-tender.

In a separate bowl, whisk together soy sauce, ketchup, rice vinegar, brown sugar, and pineapple juice. Pour over sautéed vegetables and simmer for 2 minutes.

Stir in the cornstarch slurry and cook for an additional minute until the sauce thickens and becomes glossy.

Return the seared beef to the skillet and toss to coat in the sauce. Heat together for another minute until warmed through.