Or lunch or dinner... Start with rice, Japanese short grain (sushi) rice or glutinous rice work best.
Cook it. If you have a rice cooker use it, if not, a microwave is probably the next best method, look up a guide on microwaving rice.
Fish as a main accompaniment: This can be from a tin, e.g. Mackerel or sardines, grilled on the hob in a non stick pan, oil may or may not be necessary depending on your preference/what the fish was preserved in.
Smoked fish also works nicely with hot rice, particularly; salmon, trout or mackerel, served either cold (maybe drizzled in a bit of soy sauce) or cooked, toss on a skillet or pop in the oven for 8 - 10 minutes depending on what fish you're using. If your rice is really hot, smoked fish will cook a little if you sit it on top.
If you happen to have some in you could also cook up some fresh fish too, no limits.
So many things can work, for example you could use leftover veg from last night's dinner.
One of my favs is seasoned roasted pumpkin.
Use one of the tasty kinds (not the ones we make lanterns out of), crown prince squash is cheap, easy to cook, very delicious, and often available in lidl in the autumn/winter months.
Use a sharp knife to cut in half then carefully peel (cut away) the hide before slicing in to 1 inch slices, or smaller pieces if you prefer. The peeling and cutting is the hardest part, cooking this squash is super easy though, it's hard to go wrong.
Drizzle the slices in your preferred cooking oil, some soy sauce and sprinkle some sesame seeds. Then roast for 25 - 35 minutes at 190C and your reward is a sweet, savory, umami, vegtable dish.
Kabocha (Japanese squash) is another good option. For lack of any better options you could use butternut squash, which is available at most supermarkets most of the time.
In order to make it the complete Japanese breakfast experience, the last piece is miso soup.
You can use instant, or make your own. Making your own can be a good option, as most of the main ingredients are long-life items.