Heat 1/2 cup water to a simmer in a high-sided skillet over medium-high heat. Add onions and cook about 8 minutes or until beginning to soften and brown, stirring occasionally.
Add tamari, mushrooms, and chard stems and reduce heat to medium. Cover and continue to cook about 15 minutes or until onions are soft and browned and mushrooms are tender, stirring frequently.
Add 1 or 2 more tablespoons water if onions begin to stick. Stir in chard leaves and beans, cover and cook 5 minutes or until leaves wilt. Serve in tortillas topped with tomatoes or red bell pepper.