Cut the tofu and red pepper into 1 1/2-inch triangles. In a bowl, mix the stock, soy sauce, brown sugar, garlic and mushroom soy. Heat 1 tablespoon of oil in a large nonstick skillet. Add the tofu and cook, turning once, until golden; transfer to a plate. Add the remaining teaspoon of oil to the pan; add the beans and stir-fry for 2 minutes. Add the red pepper and scallions and stir-fry until just tender. Add the tofu and Szechwan peppercorns; season with salt. Pour in the sauce and stir gently until the vegetables and tofu are evenly coated. Transfer to a platter and sprinkle with the cashews.