To marinate the steak: Using a sharp chef’s knife, slice the meat into 1/2-inch-wide strips. Add to a large bowl along with the soy sauce, lime juice, garlic, salt, black pepper, and pepper flakes. Toss to combine, then cover and refrigerate for at least 30 minutes or overnight.
Select Sauté on the Instant Pot and heat the oil. Add the onion and cook, stirring occasionally, for about 5 minutes, until the onion is translucent. Add the red pepper, jalapeños, and tomato and cook, stirring occasionally, for 3 to 5 minutes, until the vegetables are slightly tender. Transfer to a large plate, season with the salt, and set aside.
Working in batches, add the beef in a single, uncrowded layer to the Instant Pot and saute until well browned on both sides, about 2 minutes per side, without stirring; reserve any remaining marinade. Transfer the browned beef to another large plate and set aside. Press Cancel.
Pour the stock into the inner pot and, using a wooden spoon deglaze the bottom of the pot. Add half the cooked vegetables to the pot, setting the rest aside. Place the beef in an even layer on top of the vegetables, then pour in the reserved marinade.
Secure the lid and select Meat/Stew, and programme the time for 20 minutes.
While the steak cooks, make the chimichurri: In a small bowl, stir together all of the ingredients. Cover and set aside at room temperature until ready to serve.
When the cooking program is complete, perform a quick pressure release, remove the lid and using a slotted spoon, transfer the beef and vegetables to a serving platter, then cover to keep warm.
Press Cancel, and then select Sauté again. Bring the cooking liquid to a boil and cook until reduced to about 1 cup. Stir in all the vegetables and beef and cook for about 1 minute, just until heated through, and transfer to the serving platter.
Serve in tortillas with the coriander, avocado and chimichurri sauce.