This is a time of year when many people invite many other people to share a home-cooked dinner. A good make-ahead start to such a meal is soup, and a good start to such a soup is an autumn vegetable puree. Your neighbors are going to be serving pumpkin or winter-squash soup overseasoned with the spice mixture that finds its way into everything from fancy French macaroons to (I'm not making this up) artificial coffee lightener. Here's a different vegetable to puree: Celery root (celeriac), which in this version will yield a satisfyingly thick soup that tastes of almost nothing but celeriac, and with a no-less-seasonal garnish that adds an extra dimension of flavor and texture without undercutting the main ingredient's impact.
Before I describe what makes the basic soup so good, I'll give you the garnish, which (like the soup itself) can be prepared in advance. It's pears and pecans. Any time within a few days ahead, toast shelled pecans in a 325º F (165º C) oven: I do this in a frying pan for ease of stirring and handling. Warm the pan on top of the stove to give it head start, then put it in the oven; start checking the nuts and giving them a stir after ten minutes. You'll know they're done when they smell like toasted pecans and when cutting one in half reveals a lightly browned interior. Let them cool (they will become crisp as they come to room temperature), then roughly chop them. I actually cut each into four or six pieces depending on size, which takes longer than a rowdy knife attack but results in a more attractive bowl of soup. Lift the pieces handful by handful off your cutting board and put them in a bowl, leaving the tiny fragments behind for the mice (or for baking cookies).
Nearer dinner time (not longer than a couple of hours in advance), peel a couple of firm pears - two will be enough for six or eight portions of soup - and cut them into dice no bigger than 1/4 inch (6 mm). Sprinkle with lemon juice to prevent them from turning brown and refrigerate in a covered container until needed.
For the soup itself (which can be made several days ahead), peel 2-1/2 or 3 pounds (1200 to 1400 g) of celery root, taking care to cut deeply enough at the root end to eliminate pockets of soil. Cut it into chunks, adding them to a medium bowl of cold water with the juice of one lemon as you work, then simmer it in a mixture of two parts milk and one part water (from the lemon-juice-enhanced bowl), with salt and a some of the celery root's leaves (if present) until entirely tender. Discard the leaves and use a food processor or blender to puree the vegetable, adding enough of the cooking liquid to attain the consistency you like. Depending on how much waste there is (a very gnarly celery root will not yield as much clean flesh as a neater-looking one), this is likely to make enough to provide leftovers - or second helpings.
It's precisely the cooking liquid that makes this soup special: You followed the protocol and kept your peeled and cut-up celery root from discoloring by tossing it into that bowl of water and lemon juice, didn't you? And when you simmered the vegetable, you noted that the milk fell apart into white clumps clinging to the celeriac and an unappetizing-looking watery liquid that you probably couldn't wait to throw away, right?
But you didn't throw it away, did you? What happened was similar to what goes on when you make fresh ricotta using whole milk and lemon juice: The heat and acidity cause the milk solids to coagulate into those white clumps, leaving whey behind. This is a pleasant-tasting and nutritious liquid that when used in place of stock or plain milk adds gentle tartness and flavor to your soup (or to a thicker puree intended to be a side dish).
To finish the soup, warm the pecans in a skillet; add the diced pears and heat through without cooking to a mush. Sprinkle with salt and taste for seasoning. Warm up the soup, optionally adding thin slivers of celery-root leaves for their assertive flavor. Ladle out portions of about one cup (240 ml), top with pecan-pear garnish, and (optionally) drizzle with oil. Ideally, this should be toasted-pecan oil, but walnut, hazelnut, pumpkin-seed or olive oil would be fine - or no oil at all.
To me, the particular appeal of this soup is that there are no onions, no garlic, no leeks, no stock, no butter, no herbs (apart from celeriac leaves), no chilies: nothing but celery root and broken milk. And celery root has such a clear, distinctive flavor that it needs no enhancement beyond that touch of tartness from the lemon-juicy whey. And yes, you can serve it without the pear-pecan topping.
(Note: If the laws of science have lapsed in your house, which would not be surprising in the light of the recent election results, it is just possible that your milk will not curdle as the celeriac cooks. If you notice this in time, add a little lemon juice to the pan; if you don't, thin your soup with the cooking liquid as is.)