Combine coleslaw ingredients in a bowl. Let stand 10 minutes.
Pat steak dry with paper towels, then rub with grape-seed oil. Rub both sides with salt and pepper.
Heat a cast-iron skillet over high heat. Place steak on skillet and sear for 3 minutes per side. Transfer to a cutting board and let stand 5 minutes. Cut steak into thin slices.
Heat oven to 400°F. Place tortillas on a baking sheet and brush with olive oil. Bake until crispy, about 5 minutes.
Distribute coleslaw and steak among tortillas. Top with chimichurri sauce.