MAKE THE PESTO: Prepare an ice bath by filling a large bowl with ice water; set aside. Bring a medium pot of water to a boil. Add the cilantro and cook just until bright green, 1 minute. Transfer to the ice bath to stop the cooking and drain. Squeeze out as much excess moisture as possible with your hands or in a clean kitchen towel. Coarsely chop the cilantro and put it in a food processor. Add the nuts, cheese, lemon zest, and garlic. Pulse until a coarse purée forms (it should be fairly chunky). Drizzle in the olive oil and pulse until it’s the desired consistency. Set aside.
Heat the broiler. Put the cauliflower florets on a baking sheet and toss with the olive oil. Season with the salt, a few grinds of black pepper, and the thyme, if using. Broil until the cauliflower is just tender and the edges are well browned, about 10 minutes, tossing once halfway through cooking.
Remove the cauliflower from the oven and toss with about 1/2 cup [120 ml] of the cilantro-pecan pesto. (Reserve any extra pesto for another use; store, covered, in the refrigerator for up to 2 days.) Transfer the cau¬liflower to a serving plate. Sprinkle with the cotija and then the lime juice, and serve immediately.