In a small bowl, mix the stock, tamari, vinegar, garlic powder, onion powder, ¼ teaspoon pepper, and ¼ teaspoon salt. Place the tempeh in a shallow baking dish or pan, and pour the mixture over it. Gently toss to coat. Let marinate for at least two hours. You can let it marinate overnight in the fridge for more flavor.
In another small bowl, whisk together the olive oil, balsamic vinegar, mustard, garlic, and the remaining salt and pepper.
Heat a grill to medium-high, or place a grill pan on your stovetop over medium high heat. Place the tempeh strips on the grill, and cook for about 10 minutes, flipping the strips at about the halfway point.
Add the peaches to the grill, and cook them for about 6 minutes, flipping after 3 minutes. You’ll want both the tempeh strips and the peaches to have nice grill marks. Remove them from the heat.
In a large bowl, place the spring salad mix and toss with the balsamic dressing. Add the grilled tempeh and peaches, along with the tofu feta and pistachios. Serve while the tempeh and peaches are still warm.