For the flank steak and marinade: Whisk together the grapeseed oil, white vinegar, cilantro, garlic, salt, pepper, lime juice and jalapenos in a medium bowl. Put the flank steak in large casserole pan, pour over the marinade and marinate, refrigerated, overnight.
For the watermelon salad: Mix together the vinegar, honey and sugar in a bowl and stir until the sugar is dissolved. Whisk in the oil and season with salt and pepper. Add the tomatoes, watermelon and mint and toss with the vinaigrette.
For the onion rings: Heat the oil in a heavy pan until a deep-fry thermometer inserted in the oil registers 375 degrees F. Put the flour in a medium bowl. Drain the onion rings completely, and then toss in the flour to coat. Tap off any excess flour, and then fry the onions until golden brown, 2 to 3 minutes. Drain on a paper towel, and then season lightly with salt and pepper.
To cook the flank steak, preheat a grill to medium-high heat. Remove the steak from the marinade, draining off any excess. (Removal of excess marinade will prevent flaming or charring of the steak during cooking.) Grill over high heat until medium rare, 8 to 9 minutes per side. Allow the steak to rest before slicing against the grain into thin slices.
Shingle some sliced steak on each plate. Spoon the watermelon salad on top. Garnish with onion rings and cilantro sprigs.