If the nopales are not already cleaned, trim the edges and scrape off all spines and raised nodes with a small knife or vegetable peeler. Rinse well and pat dry. Leave the paddles whole for grilling. Make sure all spine points are removed fully. Even tiny ones are unpleasant in the finished salad. That thorough, careful removal at this stage is the only genuine safeguard against a truly unpleasant surprise later — no amount of careful assembly downstream can correct for a spine missed during this early cleaning.

Heat a grill, grill pan, or heavy skillet over medium-high to high heat. Lightly brush the nopales with a little oil. Grill for about 4–6 minutes per side, until they soften, their color deepens, they get light char marks, and most of the slime cooks off. Transfer to a plate and let cool until just warm or room temperature. Once cooled, dice into small bite-size pieces, about 1–1.5cm. Grill them whole first, then dice — this gives the cleanest char and the best texture, matching the style of the reference. Do not overcook until mushy. The nopales should stay tender with a slight bite. Some slipperiness is normal, but it should no longer feel slimy. That whole-paddle grilling gives each piece genuine, sustained contact with the hot grill surface before it's ever diced — cutting the nopales into small pieces first would spread that same total surface area across more, smaller pieces, giving each one considerably less real char.

While the nopales cool, dice the tomatoes, finely chop the onion, thinly slice the radishes, chop the cilantro, and crumble the queso fresco. If the onion is very sharp, soak it in cold water for 5–10 minutes, then drain well before using. Seeding very juicy tomatoes helps prevent the salad from becoming watery. That optional cold-water soak genuinely mellows the onion's sharpest, most sulfurous notes, letting it sit comfortably alongside the salad's more delicate components rather than overwhelming them.

In a small bowl, whisk together the lime juice, olive oil, dried oregano, salt, black pepper, and chopped serrano/jalapeño if using. Taste and adjust. It should be bright and limey, lightly salty, and clearly seasoned. Crushing the dried oregano between your fingers before adding it, rather than adding it whole, is what actually releases its own concentrated aroma directly into the dressing, letting it distribute far more evenly than whole, unbroken leaves would.

In a large bowl, combine the diced grilled nopales, tomatoes, onion, most of the cilantro, and most of the radish slices. Pour over the dressing and toss gently until everything is evenly coated. Let the salad sit for 10 minutes so the flavors meld. Fold in or top with the queso fresco, remaining cilantro, and remaining radish slices. That brief resting period gives the dressing genuine time to actually work through every component, letting the salad's own flavours meld together rather than simply sitting as separately coated pieces.