For the dressing: Whisk the vinegar, honey, 3/4 teaspoon salt, and pepper to taste in a large bowl. Gradually whisk in oil, starting with a few drops and adding the rest in a steady stream to make a smooth, slightly thick vinaigrette. Set aside.
For the salad: Heat a grill pan over medium-high heat. Brush the plums with olive oil and sprinkle with salt and pepper to taste. Grill the plums until slightly soft, about 1 minute per side. To get distinct grill marks, resist the temptation to move the fruit around on the pan as it cooks. Cut the plum wedges in half crosswise and toss with the dressing. Set the plums aside to marinate in the dressing for 15 minutes. (The salad can be prepared up to this point 1 hour before serving.)
Right before serving, toss the greens with the plums and dressing. Divide the salad among 4 serving plates and scatter the cheese and pecans over the tops.