For the vinaigrette, in a blender container or food-processor bowl, combine rice vinegar, walnut oil, mint, garlic, pepper, and salt. Cover, blend or process until nearly smooth; set aside.
Combine fresh spinach and toasted walnuts in a large bowl. Drizzle spinach mixture with vinaigrette; toss gently to coat. Divide spinach mixture among six chilled bowls or plates. Arrange figs and grapefruit slices on greens. Add goat cheese. Makes 6 side-dish servings.