Saute onions over high heat in 1/4 cup water until translucent. Add all other ingredients to large pot or wok. Reduce heat to medium and stir-fry until vegetables are tender but still crisp, about 10-15 minutes. Meanwhile, mix soy sauce and sugar in a measuring cup until fully incorporated. Pour over the cooked veggies, stirring to coat. Simmer another 5 minutes then serve over brown rice.