Preheat your oven to the lowest setting it will go to (mine only goes down to 170 degrees F)
If it's not already, vacuum seal the steak in a heat-safe plastic bag. I've heard of people using Ziploc bags to do this, but it's pretty hard to get all the air out and the safety of using Ziploc bags in hot water is debatable. Put the steak in a large dutch oven and cover with hot water. Check the temperature of the water using an instant read thermometer and heat on the stove as needed. The goal is to get the water just a smidge above 130 degrees (the perfect temperature of a medium-rare steak). Once it reaches the proper temperature, cover the pot with a lid and place it in the oven.
Cook for one and a half hours checking the temperature every 30 minutes, turning the oven off if the water is too hot, or turning it up if the water is too cool.
While the steak is cooking, cut the onion in half and use a mandoline to slice it very thinly into a bowl of ice water. Allow it to soak for 30 minutes then drain and add more ice water, soaking for another 30 minutes. Drain and use a salad spinner (if you have one) to dry the onions. They should be crisp, sweet, and only faintly oniony. Keep them in the fridge until the steak is ready.
Once the steak is done, remove it from the bag and salt and pepper generously. Heat a cast iron skillet over medium high heat until very hot. Add a splash of oil and sear the steak on one side (about a minute), then flip and sear the other side. Plate the steak.
Get the onions from the fridge and toss with the cilantro, lime juice, and salt and pepper to taste. Mound the onion in a huge pile on top of the steak. The steak should be enough to feed two people with a normal sized appetite, but if you use a smaller steak (or you're really hungry, double or triple the recipe)