In a small bowl, whisk the soy sauce, rice wine vinegar, sesame oil, honey, chili paste, garlic, and scallion.
Add half of the marinade to a large re-sealable plastic bag, along with the beef. Squeeze as much air from the bag as you can, seal the bag, and gently toss to coat the beef in the marinade. Refrigerate for at least 2 hours and up to 24 hours.
To the remainder of the marinade in the bowl, add the cornstarch and whisk well until dissolved. Cover and refrigerate until you’re ready to cook.
Bring a large pot of water to a boil. Add the broccoli rabe and cook until bright green and just tender, 2 minutes. Use a slotted spoon to remove the broccoli rabe and transfer to a paper towel-lined plate.
In a large nonstick skillet or wok, heat 1 tsp of the oil over medium-high heat until very hot. Remove the beef from the marinade, discard the remaining marinade, and add the beef slices to the skillet in a single layer. Cook until browned, 5 to 6 minutes. Transfer the beef to a plate.
Add the remaining 1 tsp of oil to the skillet and swirl to coat. Add the onion, bell pepper and broccoli rabe and cook until just tender, 3 minutes. Add the reserved marinade (the half with cornstarch) and toss with the vegetables. Cook, stirring frequently, until the sauce has thickened enough to coat the vegetables, about 2 minutes.
Return the steak to the pan, stir to combine with the vegetables and sauce, and remove the pan from heat.
Warm the tortillas: In a separate large skillet over medium-high heat, warm the tortillas (2 or 3 at a time) for 2 minutes, using tongs to flip them halfway.
To serve, divide the steak and vegetables among the tortillas (2 tacos per person) and serve with hot chili sauce if desired.