Pat the salmon very dry and season it with a pinch of the black pepper. Leave it out while you build the sauce so it is not fridge-cold going into the pan.
Make the caramel. Put the sugar and 1 tablespoon water in a wide pan or small clay pot over medium heat. Let it melt and bubble without stirring, swirling the pan now and then, until it turns the color of dark honey, about 4 to 5 minutes. Watch it closely at the end because it goes from amber to burnt fast.
Stand back and add the oil, then the garlic, shallot, and ginger. It will spit. Stir for 30 seconds until fragrant.
Pour in the fish sauce, soy sauce, water, and most of the black pepper. The caramel may seize into a lump; just keep it over the heat and it will melt back into the liquid as it simmers. Bring to a gentle simmer.
Nestle the salmon in skin-side down. Spoon the sauce over the top, cover, and simmer gently for 8 to 10 minutes, basting once or twice, until the salmon flakes and the sauce has reduced to a glossy syrup. Do not let it boil hard or the salmon will toughen.
Add the green onion whites and chili for the last 2 minutes. Finish with the green onion tops, a little extra ginger, and the rest of the pepper. Serve over rice with the sauce spooned on top.