Rinse the dried mushrooms with cool water and place them in a small bowl. Add enough water to cover the mushrooms and set aside for at least 30 minutes to soften.
In a medium saucepan, melt the ghee over medium heat. Add in the onions and sauté for 10 to 15 minutes or until translucent.
Then, drop in the tomato paste and fish sauce. Stir to evenly distribute the umami boosters, before dumping in the sliced mushrooms. Cook the mushrooms until the liquid is released and evaporated, about 10 minutes.
In the meantime, fish the reconstituted porcini mushrooms out of the bowl of water, and roughly chop them up.
Add the garlic to the saucepan, and cook for 30 seconds or until fragrant.
Then, mix in the reconstituted dried mushrooms, pour in the broth, and drop in the thyme sprigs. Increase the heat to high and bring the gravy to a boil.
Decrease heat to medium-low to maintain a strong simmer/low boil, and cook until the gravy has reduced by half, about 30 minutes. Be patient—you want half of the liquid to evaporate in order to concentrate the flavors. Plus, you don’t want the gravy to be thin and watery once blended.
Remove from the heat. Take out the thyme twigs and season to taste with salt and pepper.