How to Make It
In a blender or mini processor, blend the cumin with 1 tablespoon of water. Add the olive oil and blend for 3 minutes. Transfer to a bowl and let the cumin oil stand for at least 30 minutes or up to 1 day. Strain the oil through a tea strainer or a cheesecloth-lined sieve. Put the vinegar in a bowl and whisk in 1/3 cup of the cumin oil in a steady stream. Season with salt and pepper.
Heat a grill pan or large cast-iron skillet. Season the steak on both sides with salt and pepper and grill or panfry over high heat, turning once, until browned, about 3 1/2 minutes per side for medium-rare. Transfer the steak to a carving board and let rest for 10 minutes.
Pour the buttermilk into a shallow bowl and season with salt and pepper. In another shallow bowl, combine the cornmeal and flour and season with salt and pepper. Dip the tomatillo slices in the buttermilk, then coat them on both sides with the cornmeal mixture. Melt the butter in the olive oil in a large skillet. Add the tomatillos and fry over moderate heat, turning once, until golden and crisp, about 2 minutes per side. Transfer to a paper towel-lined plate and keep warm.
In a bowl, toss the mesclun with half of the cumin vinaigrette and divide among 4 plates. Thinly slice the steak against the grain. Fan the steak slices on the greens and arrange the fried tomatillos alongside. Drizzle the remaining vinaigrette over the salads and serve.