Put the doenjang and gochujang in a small bowl and mash them together with the back of a spoon until smooth and evenly blended.
Stir in the sesame oil, sugar, garlic, onion, and scallion until fully combined.
Taste it. This is the step most people skip, and it's the one that actually makes a good ssamjang — add more doenjang for depth, more gochujang for heat and color, or the rice vinegar if it tastes flat.
Stir in the crushed nuts if using, and top with sesame seeds just before serving.
Serve alongside grilled meat and lettuce or perilla leaves for wrapping. Store any extra in an airtight container in the fridge.