Prepare the vegetables. Peel potatoes and cut into cubes, you should have about 2 cups of potatoes. Dice leeks, white and green parts included. Peel and mince garlic. Roughly chop mushrooms into halves or quarters.
In a large pot on medium heat, melt butter and add diced leeks. Saute for 5-7 minutes until leeks are tender.
Add mushrooms and garlic to pot, season with a pinch of salt to help extract water, and allow to cook for another 2-3 minutes until the mushrooms have cooked down.
Add diced potatoes and thyme to the pot (stems removed), and stir well to combine.
Cover vegetables with stock, stir to combine and allow to cook 30 minutes until potatoes are tender.
Once the soup has cooked, and the potatoes are tender, using an immersion blender, or transferring in batches to a blender, puree the soup into a smooth consistency.
Depending on your potatoes, you may need to add a little more stock or water to thin the soup if it is too thick.
Season with salt and pepper to taste.
The soup can be served as is, or I like to add additional sauteed mushrooms on top. Simply heat oil in a small skillet, add diced mushrooms and saute until golden and crispy. Top the soup and add a final drizzle of olive oil before serving.