Slice tofu block into 3 long slabs lengthwise. Wrap each slab in paper towels, and press to squeeze out excess water.
Coat a large skillet with cooking spray, and then add 2 teaspoons sesame oil. Once the oil is hot, add the tofu slabs to the skillet. Fry for about 5 minutes on each side, or until delicately browned.
Remove tofu from skillet, and slice into cubes. Add the remaining teaspoon sesame oil to the skillet, and stir fry the water chestnuts, mushrooms and snow peas. Mix together water and oyster sauce, and add to the skillet along with the tofu. Cover, and cook over low heat for about 10 minutes.
