1. Season salmon with the salt and pepper. In a small bowl, whisk together the water, honey, soy sauce, mirin and garlic. Reserve.
2. Put the oil into the pressure cooker and heat over medium heat. Once the oil is hot, add the salmon, flesh side down and brown for 2 minutes. Remove pressure cooker from heat and add water mixture to the pot with the salmon.
3. Secure the lid and select Low pressure. Turn heat to high until pressure cooker reaches full pressure. Reduce temperature to maintain pressure* and set a timer for 4 minutes (for medium-cooked salmon).
4. When time expires, release pressure by selecting quick pressure release. When the red safety valve drops, remove lid and transfer salmon to a serving plate, flesh side up. Return pressure cooker to the heat, add the grated ginger, and reduce cooking liquid until it thickens. Spoon the reduced cooking liquid over salmon and sprinkle with sliced onions.
* Unit reaches full pressure when red safety valve pops up and steam releases continuously from the pressure release valve. Regulate pressure by reducing temperature to medium/medium-low to maintain full pressure. There should be slight steam and consistent hissing from the pressure release valve.
Keep a close eye on the pressure cooker when cooking. Some adjustment of temperature may be necessary to maintain pressure.