Preheat oven to 350 degrees. Put mushroom broth in a sauce pan over medium low heat with the sprigs of thyme.
Clean and prep mushrooms. In 2 separate saute pans, heat a little olive oil and 1 tbsp butter in each pan over medium heat.
In 1 of the saute pans, add half the shitake and cremini mushrooms. Add some black pepper. Don't stir the mushrooms until after 5 minutes, and then flip and let them cook for 7 more minutes. Don't salt until last minute of cooking!
Once the first half is done cooking, transfer to a plate, and add the other half of the mushrooms, adding more oil and butter to the pan.
In the other saute pan, add the chopped up portobello mushrooms. Same rules apply. Don't move around, don't salt until the end.
While the mushrooms are cooking, in a large soup pot, heat olive oil and 1 tbsp of butter over medium heat. Add the onions, carrots, celery, tomato paste, garlic, crushed red pepper flakes and salt and pepper. Let this mixture cook for about 10 minutes, until all vegetables are nice and soft.
While all vegetables are cooking on the stove top, trim the asparagus and put onto a baking sheet. Coat in olive oil and lots of salt and pepper. Put in the oven for 20 minutes.
Once onion mixture has softened, add the mushroom broth to the pot, removing the thyme. Bring to a simmer.
Carefully transfer the broth mixture to a large bowl.
Remove the asparagus from the oven. All mushrooms should be on the plate cooling at this point. You're ready to start assembling the soup.
Ladle 2 large scoops of the broth mixture and 1/4 of the asparagus to the food processor. Turn on and blend until smooth. (about 30 seconds) Return mixture to soup pot on stove over low heat. Repeat step until all broth and asparagus has run through the food processor.
Add the 1/2 cup of cream to the soup and stir to combine.
Put only the portobello mushrooms in the food processor. Hit the pulse button just once or twice. You want to get them into really small pieces, but large enough to give the soup texture. Add to the soup and combine.
Finally, add the cremini and shiitake mushrooms to the soup. Add salt and pepper to taste.
Let cook on very low heat for about 20 minutes to let flavors infuse. (It's actually best served the next day, reheated) Serve with a dallop of sour cream!