Considering how much I love the Banh Mi Ca Moi (Vietnamese Sardine Sandwich), when I came across mention of a sardine sandwich in Cheryl Lu-Lien Tan's "A Tiger in the Kitchen: A Memoir of Food and Family," I knew I had to try making it. Her friend, who is a chef, suggested cutting the fishiness of the sardines with sesame oil. I don't know if it really made that much of a difference for me since I like sardines, but you could certainly try that if you wish.
Read my recipe for Singaporean Roti John with Beef which includes the background history of the British, Malay, Indian, Singaporean influences that went into the creation of this sandwich. Since the sardines are already cooked, this version is a little quicker since you don't have to cook the meat before adding the eggs. I removed the bones and mashed the sardines with the eggs, scallions, and chili paste. Drizzled sesame oil to cook the mixture before placing the toasted bread on top. If you really want to cut the fishiness of the sardine sandwich though, I'd suggest stuffing it with slices of cucumbers and tomatoes.