Start by making the dressing. Combine in a small jar (which has a lid), the olive oil, lemon juice, honey, mustard, salt and pepper.
Place the lid on the jar and shake the salad dressing until it becomes smooth. Adjust the seasoning as needed. Set aside.
Cut the peaches into segment, I got 7-8 per peach. Make sure they are not too thin or else they will fall apart while grilling. Use a silicone brush to lightly coat each flesh side of each segment.
Preheat an electric grill to 350F or a grill pan to medium-high heat. Place the oiled segments onto the preheated grill. Cook for 1-2 minutes per side, or just long enough to get desired grill marks. Remove and set on a plate.
Assemble the salad by dividing the arugula onto 4 salad plates. For salads used as a meal, divide onto 2 larger plates. Top the arugula with the red onion, pistachios, and crumbled feta. Place the grilled peaches on top. Drizzle on the salad dressing not too long before serving to prevent any sogginess from happening. Optionally, you may want to serve the dressing on the side and let your guests choose their desired amount.