Heat a stock pot to medium heat. Add a good drizzle of olive oil .
Add in the onions and garlic. Sauté for 5-8min.
Add the celery and apple to the pot.
Sauté for another 5-8min.
Add the potato and the mushrooms to the pot followed by the pink salt.
Add the kombu to the pot followed by the water.
Turn up heat to bring to a boil, then cover and cook on medium low for at least 45min but ideally 1.5 - 2 hours.
Pour the stock into a large bowl using a sieve or colander to catch the solid ingredients. Then, use a ladle or colander to press down and extract as much of the stock as possible.
Transfer the stock to some jars. Let them cool before placing in the fridge.