This makes a smallish batch–hearty lunch servings for two, or more modest for three.
Saute the onion until translucent. Add the asparagus and fennel, add water, and simmer until everything is tender.
Meanwhile, dice the potato. In a separate pan, simmer the potato until tender. Do not drain.
Once the asparagus and onions are tender, blend them into puree. Put back in the original pot. Add the cooked potato with its water. Mash with a potato masher. (Asparagus is fibrous and I like it well blended, but potato can become gluey if put in a blender too long, which is why I use separate pots and don't blend the potato. You get a slightly grainy texture this way, which I enjoy.)
At this point the soup will not be very appealing–it somehow manages to be simultaneously bland, earthy, and grassy. It's also a drab gray-green. A little turmeric fixes the color and makes it look pale pastel green, reminiscent of spring weather. What brings the flavor to life is the acidic lemon along with the salt and fat from the cheese. Turn the heat down very low and add the lemon and cheese.
(If I wanted to make a dairy-free version, I'd try adding some olive oil and salt.)
Taste and adjust seasoning with salt, pepper, and maybe a hint of cayenne.