Fill large pot or other large, heat-resistant container with water. Insert sous vide immersion circulator and set to 130° Fahrenheit. Note: pot must be deep enough that water level falls between minimum and maximum indicators on circulator.
Place flank steak on cutting board and pat completely dry on all sides with paper towels. Generously season all sides with salt and black pepper, then place flank steak in large sealable food-safe bag.
Place garlic cloves in food processor. Pulse until garlic is finely chopped, then add fresh parsley and fresh cilantro. Continue pulsing until herbs are finely chopped, then add olive oil, lemon juice, dried oregano, red pepper flakes, and salt. Pulse again until ingredients are fully combined and chimichurri sauce forms.
Add half of prepared chimichurri to bag with flank steak. Refrigerate other half of chimichurri until ready to serve.
Remove as much air from bag as possible with vacuum sealer or water displacement method (see Notes) and seal bag completely. Once water is preheated to 130° Fahrenheit, place sealed bag in pot, making sure flank steak is fully submerged in water. Cook for 2 hours.
After 2 hour cook time, heat large skillet over medium-high heat. When skillet is warm, add neutral oil. Swirl pan occasionally to coat in oil and continue heating pan until oil is hot and shimmery.
Remove flank stank from bag and place on cutting board. Use spatula to gently scrape chimichurri off flank steak, then place steak in skillet. Sear steak 1 to 2 minutes, or until golden brown, then flip steak over and sear other side 1 to 2 minutes or until golden brown.
Transfer seared flank steak to cutting board or large plate. Set steak aside to rest 5 minutes, then carve steak into thin slices, top with remaining chimichurri sauce, and serve with desired sides.