For the marinade, combine soy sauce, mirin, water, sugar, onion, scallion, garlic, ginger, peppercorns, and dried chili pepper into a pot. Bring to a boil over medium heat then reduce to a simmer for 10 minutes. After 10 minutes, toss in the bonito flakes and mix well. Turn off the heat then allow the marinade to steep for at least 10 minutes. Strain then set aside to cool completely.
While the marinade is cooling down, prep the salmon. Slice the salmon at an angle into 1/2 inch thick pieces. Transfer to a deep plate or glass container.
Top the salmon with thinly sliced onions, red chili pepper, and green chili pepper. Pour the cooled marinade over the salmon. Place the lemon slices in one layer on top, making sure that it is submerged in the marinade.
Transfer to the fridge and allow the salmon to marinate for at least 6 hours or up to 2 days. Keep in mind that the longer it sits in the marinade, the saltier the salmon will be.
To serve, place the rice into a serving bowl. Allow the rice to come to room temperature so the heat from the rice doesn't cook the salmon. Once the rice has cooled, arrange the marinated salmon on top. Garnish with an egg yolk and sesame seeds. Serve and enjoy!