Cook the wheat berries in a pot of boiling, salted water until they are cooked but still quite crunchy, about 55 to 60 minutes. Drain and transfer to a large bowl. Meanwhile, heat 5 tablespoons of the oil in a large, heavy skillet over medium heat. Add the onions and cook, stirring frequently, until they are dark brown, about 25 to 30 minutes. Stir the onions into the drained wheat berries and cool slightly. Stir in the thyme, grapes and arugula.
For the dressing: Using a chef's knife, mash and chop the garlic on a cutting board with the salt until it becomes a paste. Transfer to a bowl and add the vinegar, pepper and remaining tablespoon of oil. Blend well and pour over the salad. Mix well, sprinkle with feta and serve.