We smash the garlic cloves carefully with the side of our chef knife and discard the peels. We sprinkle with coarse salt and mince to a paste, yielding about a tablespoon. With a paring knife, we put 6 slits around the meat and push some of the garlic in the slits. We rub the meat all over with a tablespoon of oil and rub the rest of the garlic all over the meat. Next, we rub the sazon packet all over the meat and leave in the bowl. We wash our hands well.
In a large resealable plastic bag, we combine the lemon juice with the rest of the oil. We add the meat, seal, and place in a clean bowl. We wash up and then put the bowl containing the marinating meat in the fridge overnight. The next day, we flip the bag occasionally to be sure the meat is marinating evenly.
An hour and a half or so before we want to eat dinner, we preheat the oven to 350° F. We remove the meat from the marinade, keeping the marinade bag for the gravy, and put the meat fat-side-up in an 8-inch square baking dish. We season it with salt and pepper and then put in the oven. We roast, checking after 45 minutes, every 15 minutes, until the thickest part reaches 120° F. We want to have rare to medium rare pieces of beef. This takes us 1 hour and 15 minutes.
Meanwhile, we make a gravy in a pan by sautéing some mushrooms, adding the contents of the marinade bag, and reducing the mixture for a few minutes. We add flour and water as necessary to get a good consistency for gravy and let cook slowly until the roast is done. Perhaps next time we will have measurements for this yummy accompanying gravy!
When the roast is cooked to our desired doneness, we remove it from the oven and let rest for about 10 minutes while we finish the gravy with the pan drippings.
We slice and serve.