Lightly brush both sides of tempeh slices with butter; sprinkle with salt. Heat a grill pan over medium-high heat. Add tempeh slices in a single layer; cook 1 minute on each side or until tempeh begins to brown and grill marks appear. Remove from heat. Combine 1 tablespoon soy sauce and 1 tablespoon maple syrup; brush over both sides of tempeh while still warm. Let tempeh cool completely; cut slices into bite-sized pieces.
Combine arugula, spinach, bell pepper, and onions in a large bowl. Combine the remaining 1 1/2 tablespoons soy sauce, remaining 1 1/2 teaspoons maple syrup, vinegar, oil, red pepper, and garlic in a small bowl; stir well with a whisk. Drizzle soy mixture over greens mixture; toss to coat. Place 2 cups salad on each of 4 plates; top each serving with 1/2 cup tempeh. Serve immediately.