Step 1

Crispy Onions: Whisk buttermilk and mayonnaise in a medium bowl; season with salt and pepper. Add onions and toss to coat; let sit 15 minutes.

Step 2

Meanwhile, fit a large saucepan with thermometer; pour in oil to measure 2". Heat over medium-high heat until thermometer registers 375°.

Step 3

Whisk flour and chili powder in a shallow dish; season with salt and pepper. Working in batches, remove onion from buttermilk, letting excess drip back into bowl, toss in seasoned flour, and fry, turning occasionally, until golden brown and crisp, about 4 minutes. Transfer to a wire rack set inside a rimmed baking sheet; season with salt.

Step 4

Slaw and assembly: Whisk vinegar, oil, fish sauce, honey, and tamarind in a large bowl; season with salt and pepper. Add cabbage, scallions, basil, mint, and peanuts and toss to combine.

Step 5

Serve slaw topped with crispy onions.