Prepare the potatoes:

• Peel and grate the potatoes into small parts. Place the grated potatoes in water overnight. The next day, remove the potatoes from the water and add some salt. This step helps the potatoes release excess water and ensures they become as dry as possible, with the help of a paper towel. 
• Prepare the egg mixture by whisking the eggs with mushroom garum and seasoning with salt and pepper. 
• Once the egg mixture is ready, add the grated potatoes and mix well to combine. 
• Heat a pan over medium heat and add a little oil or butter. 
• Take a portion of the potato mixture and place it in the pan. Flatten it gently with a spatula to form a thin rosti. Repeat with the remaining potato mixture, making rostis of your desired size. 
• Cover the pan with a lid to ensure the potatoes cook all the way through. Cook the rostis for about 5-7 minutes on each side, or until they are golden brown and crispy. 
• Serve the potato rostis hot, alongside the champignon sauce, the ratatouille or other sides.
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Prepare the Champignon Sauce: 

• In a separate pan, melt some butter over medium heat. 
• Add the chopped onion and minced garlic. Cook until the onion becomes translucent. 
• Add the sliced mushrooms to the pan and sauté until they are soft and browned. 
• Stir in the herbs (parsley, oregano, and chives) and season with salt, pepper, and mushroom garum to taste. 
• Pour in the cream and let the sauce simmer for a few minutes until it thickens slightly.

Serve the dish: 

• Place the potato rosti on individual plates or a serving platter. 
• Spoon the champignon sauce generously over the rosti. 
• Garnish with additional chopped herbs if desired.