Preheat the oven to 300 degrees. Put the grapes and kosher salt in a food processor and pulse until the grapes are coarsely chopped. Transfer to a strainer; let sit for a least 10 minutes. Mix the green onion, vinegar, oil, rosemary, garlic and pepper together in a small bowl. Add the grapes and mix well. ASSEMBLY: Arrange the shells on a baking sheet and fill each with 1/2 teaspoon walnuts, 1/2 teaspoon Brie, and 1/2 teaspoon grape salsa (use a fork to transfer the salsa to the shells to avoid capturing too much liquid). Bake until cheese begins to melt, about 5 minutes; do not overbake.