Slice the pressed tofu into slabs about 1.5 cm / 1/2 inch thick and pat them very dry.
Heat oil in a wide pan over medium-high and fry the tofu 2-3 minutes per side, until golden and firm to the touch.
Whisk soy sauce, gochugaru, sugar, sesame oil, garlic, and water or broth into a sauce.
Arrange the fried tofu in the pan in a single layer, scatter the onion over it, and pour the sauce around the tofu rather than directly on top.
Simmer uncovered over medium-low, spooning sauce over the tofu every couple of minutes, for 8-10 minutes, until the sauce has reduced by about half and coats the back of a spoon.
Scatter scallion and sesame seeds over in the last minute of cooking.
Turn off the heat and let it sit 5 minutes before serving — the glaze thickens further as it cools slightly.