Make the chimichurri sauce: in a food processor or blender, pulse together cilantro, parsley, onion, garlic, red pepper (if using), salt, lemon juice, lemon zest, and red wine vinegar. While the motor is running, stream in the oil and continue to blend until smooth. Set aside.
Heat a large skillet to medium high heat. Once hot, add 2 tbsp oil then the steak tips. Brown for 2-3 minutes, or until all sides are seared. Remove from the pan, but keep the pan hot.
Toss in the onion and bell pepper to the skillet. Reduce heat to medium. Cook for another 4-5 minutes, or until fragrant and browned. Add the steak back to the pan. Cook for another 2-3 minutes, until steak is cooked to your liking.
Pour in chimichurri sauce and coat the steak and veggies. Remove from heat. Serve hot over rice, cauliflower rice, or a bed of greens.