Preheat your oven to 375 degrees. Place a piece of parchment paper on a sheet pan. Once the cauliflower is separated into small pieces, toss them with 1 tablespoon of the olive oil. Simply place them on the parchment, sprinkle with salt and start roasting. It should take about 30 minutes for the cauliflower to be tender and starting to brown.
While the cauliflower is in the oven, roast the poblanos stove top. Just turn the gas burners on to medium high and place a poblano pepper directly on the grate. Turn frequently until the peppers are blistered (not burnt). Once the skin has darkened, place them in a zip lock bag. No need to seal the bag, just press it flat and let them steam. Once they steam, you'll be able to easily slide the skin off with your fingers.
Caramelize the onions. Heat a large fry pan over medium high heat. Once the pan is hot, add the remaining 2 tablespoons oil. Once the oil is hot add the onions. Turn heat down to medium low and cook until onions are golden brown. About 15 - 30 minutes. Cook time will depend on the type of pan you're using.
While the onions are cooking, remove tops, the skin and the seeds from the roasted poblano peppers. Chop the peppers into small bite-sized pieces. Add them to the caramelized onions. Stir for a couple of minutes.
Scoop the cauliflower, onion, poblano mixture into a warmed corn tortilla. Sprinkle with cotija cheese and cilantro. Provide a wedge of lime for squeezing and dig in.
How To Warm Corn Tortillas: You can't just take a corn tortilla out of the package and eat it. You must warm or heat the tortilla before filling with any taco mixture. Since we're using chunks of cauliflower, I would suggest heating the corn tortillas stove top. Steaming them would cause the tortilla not to be sturdy enough for the cauliflower mixture. Heat a heavy skillet over medium high heat. Once the pan is hot add one tortilla at a time. Once the tortilla starts to brown (about two minutes), turn over and cook the other side for about one minute more. Place the warmed tortilla under a clean dish cloth and heat remaining tortillas, keeping each tortilla warm under the towel as you finish the others.