1. Combine the ingredients for the marinade in a mortar and pestle or blender and puree until smooth. Correct the seasoning, adding salt and pepper to taste: the mixture should be highly seasoned.
2. Preheat the grill to high. Brush the steaks with adobo mixture and place on the grill. Brush the onion slices with olive oil and place on the grill. Grill the steaks to taste, 2 to 3 minutes per side for medium-rare, basting with adobo. (Rotate the steaks 90 degrees after 1 minute to create an attractive crosshatch of grill marks.) Grill the onions until nicely charred, 2 to 3 minutes per side, seasoning with salt and pepper.
3. Transfer the steaks to plates or a platter and brush 1 final time with adobo. Serve the grilled onions on the side. Serve with grilled polenta or the beans and rice.
Note: To give the steaks a richer garlic, cumin, lime flavor, marinate them in the adobo in a non-reactive baking dish for 10 minutes before grilling.
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