Begin by cutting the tempeh into 1-inch cubes and marinating it in a mixture of soy sauce, rice vinegar, and sesame oil for 10 minutes, turning occasionally.

While marinating, slice the bell peppers and onion, and mince the garlic and ginger.

Heat 1 tablespoon of olive oil in a large skillet over medium-high heat and stir-fry the marinated tempeh for about 5 minutes until golden brown.

Remove the tempeh and add the remaining olive oil to the skillet. Stir-fry the onion for 2 minutes, then add the bell peppers, garlic, ginger, red pepper flakes, salt, and black pepper, and stir-fry for 3-4 minutes.

Return the cooked tempeh to the skillet, pour over the reserved marinade, and cook for an additional 1-2 minutes until heated through.

Remove from heat, garnish with scallions, and serve hot over rice or quinoa.