Charring half the cabbage in a dry skillet lends a subtle char that contrasts nicely with the fresh crunch of the raw leaves.
Season pork with salt and pepper. Heat a large skillet (preferably cast iron) over high 5 minutes. Add half of cabbage, cut-side down; cook, flipping once, until charred, 3 to 4 minutes. Transfer to a plate. Add 1 tablespoon oil to skillet, then pork; cook, flipping once, until a thermometer inserted into thickest parts registers 140 degrees, 2 to 3 minutes. Transfer pork to a plate; let cool slightly, then thinly slice.
In a large bowl, whisk together onion, lime juice, fish sauce, chile, sugar, and remaining 1 tablespoon oil. Chop raw and charred cabbage into bite-size pieces; add to bowl along with cucumber and toss to combine. Season with salt and pepper. Divide among plates. Top with pork, mint, and peanuts; serve.