You can add a small amount of cornstarch slurry to the sauce in the pan and simmer, constantly stirring, until it thickens. I recommend using ½-1 tbsp of cornstarch with 1-2 tbsp water. For the best results, do this step after doing the initial cooking of the tofu. Remove it from the pan, thicken the sauce, and then add the tofu back into the pan to coat it.