Preheat the oven to 400°F. Place the beet wedges on a baking sheet, drizzle with 1 tablespoon olive oil, and season with salt and pepper. Roast for 30 minutes until tender, then set aside to cool slightly.
While the beets roast, brush the salmon fillets with 1 tablespoon olive oil and sprinkle with thyme, salt, and pepper on both sides.
Heat a grill or grill pan over medium-high heat. Cook the salmon for 4 to 5 minutes per side until it flakes easily with a fork.
In a small bowl, whisk the remaining 2 tablespoons olive oil with the balsamic vinegar, a pinch of salt, and a pinch of pepper to make the dressing.
In a large bowl, combine the arugula, roasted beets, and apple slices. Drizzle with the dressing and toss gently.
Divide the salad among four plates. Place one salmon fillet on top of each portion and finish with walnuts and goat cheese.