Bring the marionberries and balsamic vinegar to a boil in a small saucepan. Turn the temperature to a low simmer and place a lid on the pan. Simmer for 5 minutes, then pass through a fine sieve, being sure to press all the pulp through while leaving the seeds behind.

Peel the beet and slice it thinly on a mandolin. If it is very large, cut it into smaller pieces first. On a sheet pan or in a large bowl, sprinkle a half-teaspoon of salt over the beet slices and massage to make sure they are all exposed to it. Allow 30 minutes (or longer) for the salt to penetrate the beets. They should be limp and flimsy. Drain off any liquid, and add it to the vinaigrette.

When ready to serve, halve and core the apple, then slice it to the same thickness as the beets (alternatively, use a melon baller to make a more interesting shape). Combine the beets, apple, parsley and walnut oil in a large bowl, and season with salt and pepper. Lay the salad out fairly flat on a serving platter (as opposed to a pile), and heavily drizzle the marionberry-balsamic vinegar over the top. Top generously with Pecorino Romano and serve.