To blanch the Brussels sprouts, bring a large pot of water to a boil, and have a large bowl of ice water ready. Cook the sprouts in the boiling water for 5 minutes. Remove them from pot with a slotted spoon and place them in the ice water to keep them from cooking. After a few minutes, remove them from the ice water and blot them dry.
Whisk together the pineapple juice, garlic, ginger, tamari, agave, rice vinegar, cornstarch, tomato paste, and red pepper flakes. Set aside.
Heat the oil in large pan over medium-high heat. Add the onion and a pinch of sea salt. Cook, stirring frequently, for about 5 minutes or until the onion starts to become translucent and fragrant.
Add the red and green bell pepper, pineapple, and brussels sprouts to the pan. Cook until the peppers have softened, about 10 more minutes, stirring frequently.
Add the sauce to the pan. Gently stir to combine. Cook for about 5 more minutes, until the sauce thickens and is heated throughout. Remove from the heat.
Top with sliced scallions. Serve hot with brown rice.