Season pork steaks generously with salt and rub all over with Fat Boy Memphis Rub. Cover and let sit at room temperature 1 hour.
Prepare a grill for medium heat; oil grate. Grill steaks, turning occasionally, until thermometer inserted into the thickest part reads 145 degrees for medium, 5-7 minutes. Transfer steaks to a cutting board, squeeze a lemon half over, and drizzle with olive oil. Let rest 10 minutes.
Meanwhile, grate garlic into a large bowl and whisk with 2 Tbsp olive oil. Add mustard greens and toss to coat; season with kosher salt and pepper. Spread out greens over grate (save bowl) and grill until lightly charred and just wilted, about 45 seconds per side. Transfer back to bowl and squeeze remaining lemon half over. Add red pepper flakes and toss to coat.
Transfer greens to a platter. Thinly slice pork against the grain and place on top of greens. Drizzle any accumulated juices left on the cutting board over and sprinkle with sea salt.
