If using coriander seeds, place them in a dry skillet over medium-low heat and toast them, shaking the skillet back and forth, until quite fragrant, about 6 minutes. Transfer the seeds to a plate to cool. Dump the seeds into a mini food processor or spice grinder and coarsely grind them, or crush them with the flat side of a chef’s knife. Transfer the pulverized seeds to a small bowl.
Bring a small saucepan of salted water to a boil. Dump in the green beans and cook for 1 minute. Drain and run under cold water to cool. Pat the beans dry, then very thinly slice the beans crosswise into small rounds (or, for a somewhat fancier schmancier presentation, halve the beans lengthwise into long slivers), and transfer the beans to a large bowl. Add the cabbage and 1 tablespoon cilantro and toss to combine.
In a small bowl, whisk together the fish sauce, 2 tablespoons lime juice, garlic, and sugar.
In a large skillet over medium-high heat, heat 1 tablespoon oil. Add the beef (or pork or chicken) and 1/4 teaspoon salt and cook, stirring with a wooden spoon and breaking up the chunks of meat until it's cooked through, about 4 minutes. Remove the skillet from the heat and add the remaining 2 tablespoons cilantro, the fish sauce mixture, the onion, scallions, cashews, basil, mint, cayenne, and half the sliced chile and stir to combine. Taste and adjust the seasoning if necessary.
To the cabbage mixture, add the remaining tablespoon lime juice, the remaining tablespoon oil, and 1/4 teaspoon salt. Toss to combine.
Divide the cabbage mixture among 4 plates and top with the meat mixture. Sprinkle with the basil leaves and the remaining chile and coriander seeds, if using. Instruct guests to eat the Thai beef salad with their hands. (Although you could offer a fork and it wouldn't be the worst thing in the world.)