Bring all the marinade ingredients to a boil over medium high heat and briefly boil for a minute or two. Cool completely.
Thinly slice the onion, chili pepper(s), and lemon.
Cut the salmon fillet into small pieces (about 1/4-inch thick and 2 to 3 inch long).
Place the salmon in an airtight container. Add the chili pepper, onion and lemon slices. Keep it in the fridge while waiting for the marinade to cool.
Pour the marinade over the salmon (see note 2). Refrigerate. The salmon is ready to eat after 4 to 5 hours, but it will be better the next day and best to consume it within 2 days.
How to serve: You can simply serve it as a side or main dish with a bowl of rice. Or, serve it on top of rice as a rice bowl (deopbap). Garnish it with optional thin slices of fresh onion and/or microgreens or baby sprouts. Spoon some marinade over the salmon and rice. Serve wasabi on the side if you like.
You can also throw in a small piece of dasima (dried kelp), peppercorns, and/or some garlic and ginger slices if you want. Boil a little longer (5 min) if you're adding these ingredients.
Depending on the size of the container, you may have a little extra marinade. Keep it in the fridge and use it as a sauce for stir-frying or as a dipping sauce.