Heat butter in a large heavy-bottomed pot over medium heat until foaming. Add the diced onion and minced garlic and cook for 4 to 5 minutes, stirring occasionally, until the onion turns translucent and the kitchen smells fragrant.

Add the sliced mushrooms to the pot. Cook for 8 to 12 minutes, stirring occasionally, until the mushrooms release all their liquid and the edges develop golden-brown color. Do not rush this step. Never rinse mushrooms under running water; wipe with a damp paper towel so they brown properly rather than steam.

Pour in the vegetable broth and add the thyme. Bring to a boil, then reduce heat to medium-low and simmer uncovered for 15 minutes until the broth deepens in color and aroma.

Remove the pot from heat and let cool for 2 to 3 minutes. Using an immersion blender, pulse 4 to 6 times to partially break down the mushrooms while keeping some chunks for texture. If using a countertop blender, work in small batches, fill no more than halfway, and keep the lid slightly vented to release steam safely.

Add the full-fat cottage cheese directly to the pot. Blend for 30 to 45 seconds until completely smooth and creamy with no white curds visible. Taste and adjust salt and pepper. Ladle into bowls and garnish with fresh parsley.