In a small casserole dish, arrange the steak in a single layer. Sprinkle some sazon, adobo seasoning, and lime juice. Add some sliced onion pieces on top. Layers should be repeated until the entire steak is seasoned. Over the mixture, pour the beer. For at least an hour and up to 24 hours, cover and chill.
Set the grill to medium-high heat.
Eliminate marinade. For medium-doneness, grill the steak for 6 to 7 minutes on each side, until the exterior is gently browned but the inside is still slightly tender.