In a pot, combine soy sauce, mirin, water, sugar, onion, scallion, garlic, ginger, black peppercorns, and optional dried chili pepper. Boil the mixture, then let it simmer for 10 minutes to meld the flavors.

Stir in bonito flakes after 10 minutes of simmering. Turn off the heat, let it steep for another 10 minutes, then strain and cool the marinade.

Slice the sashimi-grade salmon at a 45-degree angle into 1/2-inch thick pieces.

Place the salmon slices in a deep dish, topping them with sliced onions and chili peppers. Pour the cooled marinade over the salmon and layer with lemon slices.

Cover the dish with plastic wrap and refrigerate for at least 6 hours, or even up to 2 days for optimal flavor infusion.

Prepare your cooked rice and let it cool down slightly. Serve the marinated salmon over the rice, adding optional garnishes like egg yolks and sesame seeds.