Steam the tempeh cubes in a steamer basket over boiling water for 10 minutes. This removes any bitterness and helps the tempeh absorb the sauce better.
While tempeh steams, whisk together soy sauce, rice vinegar, maple syrup, sesame oil, and cornstarch in a small bowl. Set aside.
Heat vegetable oil in a large wok or skillet over medium-high heat. Add the steamed tempeh and cook for 5-6 minutes, stirring occasionally, until golden brown on all sides.
Add the broccoli and bell pepper to the pan and stir-fry for 4-5 minutes until vegetables are tender-crisp.
Add the minced garlic and ginger, cooking for 30 seconds until fragrant.
Pour the sauce over the tempeh and vegetables, tossing everything to coat. Cook for 2-3 minutes until the sauce thickens and becomes glossy.
Remove from heat and garnish with sliced green onions and sesame seeds. Serve immediately over rice or noodles.