Karashi in the mayonnaise is the smallest possible change you can make to a tamago sando, and it is also one of the most effective. The build is otherwise the standard one, mashed hard-boiled egg between soft trimmed shokupan, but the kewpie binding the filling is spiked with karashi, the sharp Japanese mustard, so a thread of clean nasal heat runs underneath the familiar cool creaminess. Karashi is not a condiment that sits on the tongue the way Western mustards do; it hits high and fast in the sinuses and then clears, which means it cuts the richness of the egg-and-mayo mass without leaving a lingering bitterness. The sandwich still tastes like a tamago sando. It just has a spine.
The craft is calibration, because karashi is potent and the margin for error is narrow. The egg is hard-cooked and chopped or pressed in the usual way and bound with kewpie, but the mustard is whisked into the mayonnaise before it meets the egg so the heat is distributed evenly rather than landing in hot pockets. The quantity is the whole game: enough that the warmth is unmistakable and the filling reads as deliberately seasoned, not so much that it overwhelms the mild egg and turns punishing. The shokupan stays soft and untoasted, its sweetness now playing against the mustard's bite as well as the egg's tang. A good one is balanced, the karashi announcing itself as a clean rising heat that fades before the next bite, the filling still glossy and cohesive and spread corner to corner. A sloppy one either buries the mustard so far that the upgrade is invisible, or overshoots so the heat scorches and the egg disappears behind it, or distributes it unevenly so one bite is bland and the next is harsh. The bind is the same structural job as the plain version, holding the slices and staying creamy without weeping, with the extra demand that the mustard be fully emulsified into the mayo so the seasoning is uniform across the whole filling.
That keeps this in the seasoning-tweak branch of the tamago family, a sandwich about adjusting the dressing rather than reworking the egg or the bread. The variations move along the heat source and its intensity: some push the karashi harder for a genuinely bracing build, some round it with a little honey or extra mayonnaise to soften the edge, some swap toward wasabi for a greener, more volatile sharpness or fold in black pepper for a duller, warmer heat instead. Each of those changes the character of the bite in a distinct direction and deserves its own article rather than being crowded in here.