Marinate the salmon with Japanese BBQ sauce or any other seasoning you prefer for 20 minutes. Pat dry the skin side with a paper towel.
Preheat the cast iron pan until it feels very hot when you hold your palm above it. Move the pan aside and let it cool down for about 30 seconds (to prevent the food from burning), then add 2 tbsp of oil. Return the pan to the stove and place the salmon skin-side down over medium heat. Cook for about 3 minutes.
Use a spatula to gently detach the fish skin from the pan, then place the salmon on a plate for later use. (No need to flip it during the cooking process; the flesh side is still uncooked, and we will finish the cooking process with other ingredients later.)
In the same pan, add garlic and sauté with potatoes. Turn the heat to medium-high and add asparagus, season with salt, and mix well with everything. Place the salmon on top, with the flesh side down, add a splash of water (about 1/4 cup), and quickly cover it with the lid. Let it cook for about 10 minutes.
After 10 minutes, turn off the heat and season with black pepper or any other seasoning you prefer. Enjoy!