1/2 cup fresh lime juice
1/3 cup water
1/4 cup fresh orange juice
1 1/2 teaspoons salt
1/2 teaspoon oregano
1/2 teaspoon freshly ground pepper
1/2 cup olive oil
8 large garlic cloves, thinly sliced crosswise
1 teaspoon ground cumin
6 thin sirloin steaks (8 ounces each)
3 tablespoons chopped fresh cilantro
Vegetable oil, for the grill
2 sweet onions, sliced 1/4 inch thick
Mix the lime juice, water, orange juice, salt, oregano and pepper in a bowl.
Heat the olive oil in a deep saucepan over moderate heat. Add the garlic and cumin and cook until the garlic is fragrant and golden, about 2 minutes. Very carefully add the juice mixture and bring to a boil. Remove from the heat and let the mojo cool to room temperature.
Arrange the steaks in a large glass baking dish. Add the cilantro to the mojo and pour half of it over the steaks; turn several times to coat evenly. Cover and marinate in the refrigerator for 1 to 2 hours. Reserve the remaining mojo.
Light a grill. Lightly brush the grate with oil. Remove the steaks from the marinade and pat dry. Brush the onion slices with some of the reserved mojo. Grill the onion slices over the cooler part of the grill, until lightly charred and tender, about 4 minutes per side. Grill the steaks over high heat until medium rare, 2 minutes per side. Transfer the steaks and onions to a platter; spoon the remaining mojo on top and serve.