Slice the mushrooms, dice the onion and grate or finely chop the garlic.
First, start by sautéing the mushrooms. Heat up about 1 tablespoon on olive oil in a pot on medium to high heat. Add part of the mushrooms; enough to cover the bottom of the pot, but not so many that they stack on top of each other.
Allow the mushrooms to sauté for a few minutes, turning them occasionally. They will develop a golden hue, and shrink significantly. Remove the mushrooms, set them aside, and repeat the process until all the mushrooms are sauteed. Add some olive oil when needed to prevent the mushrooms from sticking to the pot.
Add the onions to the now empty pot. Sauté for a few minutes until it’s soft and translucent. Then, add the garlic and sauté for another minute.
Add the cooked mushrooms, all the seasoning, oat milk and vegetable broth. Cover the pot with a lid, and leave the soup to simmer on medium heat for about 10-15 minutes.
In a small bowl, combine the cornstarch and cold water, and stir until you have a thin paste, called a cornstarch slurry. Add this to the soup, and leave the soup to simmer for another 5 minutes. This will allow the soup to thicken up.
If you prefer a thick and creamy soup, you can use an immersion blender to blend the soup as smooth as you like. I like to blend about half of the soup, and leave half the mushrooms whole for some extra bite.