In a large pot, melt the butter over medium-high heat. Add the sliced mushrooms and a pinch of salt and pepper. Sauté the mushrooms, stirring often, until the liquid has almost evaporated and the mushrooms got some color (about 12 min).
Add the finely diced onion to the mushrooms. Sauté, stirring, until the onion has softened (about 2-3 minutes). Be careful not to burn the onion. Reduce the heat if needed.
Add the minced garlic, Italian-style dried seasoning, and lemon zest. Mix it all in for about 20-30 seconds.
Add the flour, stirring it in, toasting it for a few seconds.
Add a cup of stock to the pot, stirring and scraping the bottom (to deglaze and dissolve the fond formed at the bottom of the pot). Add the rest of the stock, bring to a simmer, and simmer for 15 minutes. Reduce the heat as needed to make sure the soup is simmering not boiling hard.
Once the mushrooms are soft, remove the pot from the heat and blend the soup.
Once the soup is blended, add the cream. If you’ve used a countertop blender, put the soup back in the pot (carefully). Bring the soup to a simmer and turn off the hob/take off the heat when it just starts boiling (don’t let it boil).
Once the pot is removed from the heat, add lemon juice, parmesan, and chopped sun-dried tomatoes, and stir everything in. The soup is now ready to be served.