Heat the oven to 400°F (200°C). Line a baking tray with parchment paper, then separate the head of cauliflower into bite-size florets so they roast evenly.

In a large bowl, toss the cauliflower florets with olive oil and the package Old El Pasoᵀᴹ taco seasoning mix until every piece is well coated. Transfer the seasoned cauliflower to the prepared baking sheet in a single layer and roast for 25 to 30 minutes, rotating the baking sheet once halfway through, until the cauliflower is tender and cooked through with nicely browned edges.

Meanwhile, prepare the package Old El Pasoᵀᴹ hard and soft taco dinner according to the package directions so the shells are warm and ready to fill. Layer each taco shell with shredded lettuce, roasted cauliflower, crumbled feta cheese, pickled red onion, and fresh cilantro sprigs. Serve immediately with a sliced lime wedge on the side for squeezing over the tacos just before eating.