2largered onionspeeled and sliced into 1/2-1 inch rings
1 1/2poundsflank steaktrimmed
Preheat grill on high heat.
Brush 1 tablespoon olive oil on both sides of onion slices, keeping rings intact.
Grill onions, flipping as needed, until both sides are charred and onions are soft, about 30 minutes. As the outside rings of onion become charred, removed them and continue to grill the remaining onions. Work carefully, because the more you flip the onions, the more they will fall apart.
Cool onions and dice finely.
Pour vinegar in a small sauce pan. Bring to a boil and simmer for 1-2 minutes till it thickens somewhat to make a glaze.
Combine vinegar, thyme and chili flakes with onions.
Preheat grill on medium-high heat.
Brush 1 tablespoon olive oil on both side of the steak; season with salt and pepper.
Grill steak until the first side is nicely grilled and starting to char; flip and continue to cook to medium doneness, about 140 degrees, or more for desired doneness.
Remove from grill and let rest for 5-10 minutes before slicing.