For the dressing: In a large bowl whisk together the honey, vinegar, extra-virgin olive oil and 1/4 teaspoon salt until emulsified. Set the bowl aside.
For the salad: Drizzle the plums with the olive oil and season with 1/4 teaspoon salt. Place the plums on the hot grill and cook until golden brown all over, about 1 minute per side. Remove from the grill and let cool slightly before cutting into wedges.
Add the radicchio and endive to the bowl with the dressing, season with 1/4 teaspoon salt and toss well to coat. Add the plums and toss gently. Sprinkle with the feta and pistachios.