Lay a 10- to 12-inch-long sheet of foil on a heatproof surface and rub lightly with oil.
In an 8- to 10-inch frying pan over medium-high heat, combine sugar and walnuts. Shake and tilt pan often until sugar is melted and amber-colored and until nuts are coated, 4 to 5 minutes total. At once pour walnut candy onto foil. Let candy cool until hard, 12 to 15 minutes. Discard foil; break candy into chunks, put in a heavy plastic food bag, and hit gently with a flat mallet to break into small pieces. If making ahead, store airtight up to 2 days.
In a wide shallow bowl, whisk vinegar with oil, honey, mayonnaise, and pepper.
Add lettuce, onion, raspberries, cheese, and walnut candy. Mix and season to taste with salt.