Prepare the marinade: In a medium bowl, combine the soy sauce, paprika, salt, black pepper, minced garlic, minced ginger, tomato paste, and brown sugar. Stir well.

Marinate the steak: Add the sliced flank steak to the marinade, tossing to coat evenly. Allow it to rest for 10-15 minutes to let the flavors meld together.

Make the slurry: In a small bowl, whisk together the water and cornstarch until smooth. Set aside.

Heat the oil in a large skillet or wok over medium-high heat. Add the marinated steak, spreading it out in an even layer, and cook for 2-3 minutes on each side until browned.

Cook the vegetables: In the same skillet, add a bit more oil if needed, then add the sliced onion and bell peppers. Stir-fry for 3-4 minutes until they begin to soften but still retain some crispness.

Combine everything: Return the steak to the skillet with the vegetables. Pour in the cornstarch slurry, stirring constantly as the sauce thickens and coats the beef and vegetables, about 2-3 minutes.

Serve hot over rice or noodles. Enjoy!