1. In a blender, combine oil, lime juice, orange juice, basil, garlic, cumin, salt and pepper. Blend until very smooth.
2. Pour the marinade over the steak. Marinate at least 1 hour, or up to 24 hours. Try to flip the meat a few times while  it marinates. Don't marinate the meat more than 24 hours, or the citrus will make the meat overly soft and mushy. 
3. Shake most of the marinade off of the steak right before cooking.
4. Flank steak can be cooked in a hot skillet, on the grill, or under the broiler. Don’t cook it too long - the longer flank steak cooks, the chewier it gets. Rare or medium-rare is best.
5. The steak should reach a temperature of 130 F, then rest for 5 to 10 minutes under foil before slicing it against the grain. While the meat rests, the temperature should rise to about 135 F (medium rare).
6. Sprinkle sea salt on the sliced steak before serving.

Cooking Methods

Gas Grill: Heat the grill on high. Sear flank steak for 2 minute on each side, then reduce heat to medium and continue to grill 9 to 12 minutes, turning occasionally

Oven Broil: Turn the broiler on high, and let it heat up for at least 5 minutes. Place the flank steak in a broiler pan and broil about 4 inches from the heat, usually for about 10 to 15 minutes. Flip and turn once or twice while it cooks. 

Pan Seared: Heat a cast iron skillet for about 5 minute over high heat, until very hot. Add a generous drizzle of oil or butter and add the steak. Sear for 10 to 14 minutes, flipping once or twice.