Pull the plastic off the tofu container (you may need to cut it with a knife) and tip the liquid out of it. Put the tofu on the cutting board and cut it into 4 equal pieces, then cut each of those pieces in half the other direction to end up with 8 pieces.
Now put a dish towel or 2 layers of paper towel on the cutting board and put the tofu on top. Let drain at least 20 minutes (and up to 2 hours), then put the dish towel in the wash or the paper towels in the trash.
Put the soy sauce, water, ginger, garlic, sesame oil (if you like it), and chili pepper flakes (if you want to make it spicy) in a small pot.
Put the pot on the stove and turn the heat to high. When it comes to a boil, lower the heat to low and cook for 10 minutes. The sauce should have halved and be thicker than it was. Set the sauce aside.
Put the skillet on the stove, turn the heat to medium, and carefully add the oil.
Add the tofu and cook until it is golden brown underneath, about 3 minutes. Use the spatula to turn the tofu over and cook until golden, 3 more minutes.
Add the sauce to the pan and cook 1 minute.
Serve right away, sprinkled with sesame seeds and scallions.