In a shallow pot set over low to medium heat, add in the vegetable oil and onions. Sprinkle with 1/2 teaspoon of salt. The goal is to cook to the onions slowly until it starts to caramelize on the edges. This process takes time. Stir constantly for about 15-20 minutes.
Add in the garlic, celery, and carrot into the pot. Stir until evenly distributed. Cook until celery starts to become translucent. This takes about 5 minutes.
Pour in the mushroom as well as all the dried herbs and remaining salt. Stir to distribute evenly. Cook on low heat, stirring frequently. Don't be tempted to season the mushroom to taste since this paste goes into other dishes. You want it to be undersalted so that you can use it as you desire.
The mushroom paste is ready when mushroom stops releasing liquid in the pan. It shrinks to about 1/3 to 1/4 of its original volume in the pan. This takes between 15-25 minutes.
Store in a fridge in a tightly closed container for a couple of weeks or in a freezer for up to 4 weeks.
