Wash your mushrooms in cold water first. Slice them into thin and even pieces now. Dice your yellow onion into very small bits.

Place your large pot on the stove top. Turn the heat to a medium setting. Add the unsalted butter to the hot pot. Let the butter melt until it starts bubbling.

Add the diced onions to the melted butter. Cook them for about five minutes total. They should look clear and feel very soft. Do not let the onions turn brown.

Toss all the sliced mushrooms into the pot. Stir them well to coat with the butter. Cook them until they release their natural juice. They will shrink in size during this step.

Sprinkle the flour over the cooked mushroom mix. Stir it constantly for two full minutes. This removes the raw taste of the flour. It will look like a thick brown paste.

Add the chicken broth to the pot slowly. Whisk the liquid while you pour it in. This helps prevent any flour lumps from forming. The liquid should look smooth and thin now.

Bring the liquid to a very light boil. Turn the heat down to a low simmer. Let it cook for twenty minutes without a lid. The soup will start to thicken up slowly.

Turn off the heat for a short moment. Use your immersion blender inside the large pot. Blend until the soup is mostly smooth now. Leave a few chunks for a nice texture.

Pour the heavy cream into the warm soup. Stir it gently with your wooden spoon. The soup will turn a beautiful light color. Do not let the soup boil again now.

Add the sea salt and the black pepper. Taste a small spoonful of the warm soup. Add more salt if it needs more flavor. Add the thyme and lemon juice at last.

Turn the heat to low for two minutes. This ensures the cream is nice and warm. Stir it one last time to mix everything. The soup is now ready for your guests.

Ladle the soup into deep ceramic bowls. Garnish with a fresh sprig of green thyme. Serve it with a side of crusty bread. Enjoy your home made French restaurant meal today.