Preheat oven to 425 degrees F and line a large baking sheet with parchment paper.
Place the cubed eggplant on the baking sheet and drizzle with olive oil and sprinkle with salt and pepper. Gently toss to combine (the eggplant will not be coated in oil since it is such a great absorber). Roast the eggplant until tender and golden brown, approximately 25 minutes.
For the onions, heat a large skillet over medium heat. Add butter to the skillet followed by the onions, trying not to crowd the pan. Mix well to coat. Reduce heat to medium low and continue to cook your onions for approximately 40 minutes, stirring every 5 minutes, until the onions are deeply browned.
Cook your farro according to package instructions (I used quick cook farro from TJs).
Once farro has finished cooking, drain any additional liquid and immediately add the spinach. Toss the spinach with the farro as the heat and steam will help wilt the spinach.
Once the eggplant and onions have finished cooking, add them to the farro and spinach along with the chopped parsley, pistachios, grapes and feta. Gently toss everything together to combine.
To prepare the dressing add the walnuts, olive oil, white wine vinegar, water, dijon mustard, honey and a pinch of salt and pepper to the bowl of your blender and blend until smooth. Season to taste and add additional water to thin, if desired.
Divide salad between desired number of bowls and drizzle with the dressing.