Add the remaining 1 teaspoon of oil to the pan along with the onion. Cook, stirring frequently, until it begins to brown, about 5 minutes. Add the bell peppers and the pineapple to the pan.
Whisk together the tomato paste, maple syrup, cornstarch, remaining 2 tablespoons tamari, 2 tablespoons vinegar, and ½ cup of the reserved pineapple juice in a small bowl.