Drain the tofu block and pat it dry on both sides with paper towels.
Slice the tofu crosswise into slabs about 1 cm (1/2 inch) thick.
Squeeze the kimchi lightly over a bowl, saving 2 tbsp of the brine, then cut it into bite-size pieces.
Bring a wide pan of water to a gentle simmer, slide in the tofu slabs, and heat for 4 to 5 minutes until they are hot all the way through; lift them out with a slotted spatula and drain.
Alternatively, pan-fry the slabs in 1 tsp oil over medium heat for 3 minutes per side, until the surfaces firm up and turn pale gold.
Heat 1 tbsp oil in a skillet over medium-high heat until it shimmers.
If using pork belly or bacon, add it now and cook for 3 to 4 minutes, until the edges are browned and the fat has rendered.
Add the chopped kimchi and stir-fry for 4 minutes, until it darkens from bright red to a deeper brick color and stops smelling raw.
Stir in the sugar, gochugaru, and reserved kimchi brine, and cook 2 minutes more until the liquid mostly cooks off and the pan sizzles rather than bubbles.
Taste and add a pinch of salt only if the kimchi was mild.
Turn off the heat and stir in the sesame oil.
Fan the warm tofu slabs around the edge of a plate and pile the kimchi in the center.
Scatter scallion and sesame seeds over the top and serve immediately, while the tofu is still hot.