Preheat an oven to 375F (190C).

Butterfly the flank steak by cutting into it lengthwise using a sharp, long knife. Cut through, with the grain almost in half without completely separating it.  Open the steak like a book and spread it out.

Season the steak on both sides with salt and pepper.

Place the steak cut side up with the wide side facing you. Sprinkle with the garlic pieces, cumin and oregano. Cover with parsley and cilantro.

Arrange onions, carrots and eggs in two, neat vertical rows.  Place a layer of spinach over each row.

Roll up the steak like a jelly roll, with the rain of the meat running the length of the roll. Tie it in place using butcher's twine in 3 or 4 places.

Heat olive oil in a roasting pan or Dutch oven large enough for the steak roll.

Brown the steak on all sides, for about 15 minutes.

Transfer the pan to the oven and roast for 1 1/4 to 1 1/2 hour, or until steak is tender to the touch.

Remove from the oven.

At this point, you can either transfer the matambre to a cutting board for slicing if you want to eat it now, or to a baking dish if you want to chill it overnight.

In either case, let meat rest for 30 minutes beforehand since it will be tough out of the oven.

If chilling overnight, weight it down with a heavy object places on a plate and chill overnight in a baking dish.

Remove from the fridge, slice into 1/2 or 1 inch pieces and serve it at room temperature.