Step 1: Make the dressing: Place the chiles, garlic, sugar, and black pepper in a large serving bowl and mash to a paste with the back of a wooden spoon. Whisk in the lime juice and fish sauce.
Step 2: Make the slaw: Peel the mango (wear rubber gloves if you have sensitive skin—mango sap can cause a poison ivy–like rash) and cut carefully the flesh off the seed. Cut the mango into thin slivers.
Step 3: Shred the cabbage in a food processor fitted with a shredding disk or thinly slice it by hand.
Step 4: Add the mango, cabbage, green beans, and cilantro to the dressing. Taste for seasoning, adding more sugar and/or lime juice as necessary; the slaw should be highly seasoned. Sprinkle the peanuts over the slaw and dig in.