Heat 1 tablespoon of the oil in a 4-5 quart Dutch oven or stock pot over medium until shimmering. Add the onions and fennel and saute until softened, about 5 minutes.

Scootch the vegetables to the side, and add the remaining oil to the cleared spot. Spoon the cilantro stems, curry powder, turmeric, and a pinch of salt over the oil, and stir in place to create a fragrant paste.

Add the vegetable broth and cauliflower, and bring the soup to a gentle bubbling boil over medium-high heat, then immediately reduce heat to medium-low and simmer for 15 minutes. Test the cauliflower florets by slicing one with a knife: it should cut easily through. Stir the mustard into the soup.

Puree the soup thoroughly: use an immersion blender directly in the pot to blend the soup smooth, or use a regular blender (you'll need to do it in batches), and return the soup to the pot.

Add the coconut milk, cilantro leaves, dill, and tarragon. Stir thoroughly.

Let the soup heat gently over medium-low for 10 minutes - if the soup begins to bubble at any point, reduce the heat a bit.

Squeeze the lemon half over the soup, and add a pinch of salt with a few grinds of black pepper. Stir, and taste, adding more salt as needed.

Serve immediately with a loaf of crusty bread. This soup can be made ahead of time and stored in the fridge. Leftovers, if there are any, will taste great for lunch the next day.