Place tofu cubes on several layers of paper towels. Cover with additional paper towels; let stand 5 minutes.
Heat oil in a large nonstick skillet over medium-high heat. Add tofu to pan; cook 7 minutes or until lightly browned, gently turning occasionally. Remove from pan; keep warm. Add snow peas, bell pepper, 1/4 cup water, onions, ginger, and garlic to pan; stir-fry 3 minutes. Add mushrooms; stir-fry 2 minutes.
Combine remaining 1/2 cup water, tamari, peanut butter, cornstarch, Sriracha, and salt in a small bowl; stir well. Add tamari mixture and tofu to pan; cook 1 minute or until thickened, stirring constantly.