Toss the scallions in a bowl with 2 tablespoons olive oil and sprinkle with salt and pepper. Grill the scallions until the bulbs are tender and the greens are well-charred, about 8 minutes. Set aside to cool. (Alternately, you can char the scallions in a cast-iron pan on the stove.) Chop the white and light green parts finely, then discard the remaining ends of the scallions.
Bring a large pot of salted water to a boil for the orzo. Drop in the orzo and begin to cook. Meanwhile, add the chopped scallions to a medium bowl. Add the vinegar, Dijon mustard, remaining 1/3 cup olive oil and half of the olives and whisk until combined. Season with salt and pepper.
When the pasta is finished cooking, drain and rinse to cool. Immediately toss in some of the vinaigrette, along with the cucumber, tomatoes, herbs, red onion and remaining olives. Mix well. Add more dressing to taste. Top with the goat cheese and serve.
This salad goes well with grilled proteins, like shrimp or chicken.