Preheat grill to high heat. Dry roast cumin seeds until fragrant and toasted (1 minute), set aside to cool slightly, then coarsely crush with a mortar and pestle. Place half in a bowl and reserve for the chismol. Place remainder in a container with garlic, oregano and oil and season to taste. Add steaks, turn to coat and set aside while you prepare the chismol.
For chismol, halve tomatoes, squeeze out the seeds (discard), then dice tomato and place in a bowl. Add onion, red pepper, lime juice, oregano, reserved cumin, 2 tbsp cold water and 1 tsp sea salt flakes and mix to combine.
Add lime juice to steaks, turn to coat, then transfer to grill and grill, turning once, until caramelized and cooked medium-rare (8 minutes). Set aside to rest (5 minutes), then thinly slice.
Heat a cast-iron frying pan and heat tortillas over high heat, turning occasionally, until golden and warmed through (20-30 seconds each side). Wrap in a tea towel to steam lightly and keep warm. Serve with steak, chismol, feta, radish and cilantro, and serve with lime halves and extra chismol.
