Spray a heavy nonstick skillet with cooking spray and place over high heat. Add tempeh and onion; cook and stir until onion softens, about 5 minutes. Add squash and tomatoes; cook and stir until squash begins to soften, 5 to 8 minutes.
Stir balsamic vinegar, lime juice, and agave nectar into the skillet. Toss until mixture is well coated. Season with pepper and salt. Add arugula; toss until it begins to wilt, 1 to 2 minutes.