Mentai toast is what happens when Fukuoka's love of mentaiko meets the toast rack. A thick slice of soft white bread is spread with a butter worked through with mentaiko, the spicy marinated cod roe that the city is known for, then toasted or grilled so the butter melts into the crumb and the roe's tiny eggs crisp at the edges. It is open-faced, eaten hot, more a savory breakfast or kissaten plate than a closed sandwich, and its whole identity is one bold spread on one plain canvas. The bread is a frame; the mentaiko butter is the entire reason for the order.
The craft is in the spread and in the toast carrying it. Mentaiko is mashed or loosened from its membrane and beaten into softened butter, sometimes with a little mayonnaise or soy for roundness, until it is a pale coral paste flecked with roe and carrying a salty, savory, gently chili heat. It is spread thick to the edges of a slice of soft shokupan, then put under heat: melted butter soaks the crumb while the surface roe toasts and turns nutty and slightly crisp. The skill is in heat and spread depth being matched. Too little roe and it reads as plain salty butter; too much and the slice goes oily and overpoweringly fishy. Too hot and the eggs scorch bitter; too cool and the butter never melts in and the toast stays pallid. Done well the eat is a crisp-edged toast, a rich savory butter driven into the crumb, and the roe's salt and faint heat lifting it. Done poorly it is greasy, dull, or burnt, the roe wasted on soggy or scorched bread.
Eating it is meant to be quick and savory, a hot single slice with coffee or as a small plate rather than a packed sandwich, the kind of thing that reads as a regional breakfast treat. The heat from the mentaiko is gentle, more warm savor than real spice, which is part of why it works at any hour.
The variations mostly add to the slice. Some scatter shredded cheese over the mentaiko butter and grill it so the top browns and pulls; some lay shiso or nori under or over it; some finish with a brush of soy or a squeeze of lemon. There is also a closed sandwich form, the Hakata mentaiko sando, where the same roe goes between slices rather than open-faced, eaten cold rather than hot. That closed Hakata sando build deserves its own article rather than being crowded in here.