brussels sprouts & chestnuts in brown butter sauce

as i type this, it's snowing outside. for portland, this is a very, very big deal. two years after the fact, i'm still hearing stories about the "snowpocalypse" of 2008, during which there was just under a foot of snow and the entire city shut down for several days. people were "buried in their houses," all public transportation stopped running, and i've heard multiple accounts from friends who were trapped in the airport for days on end about being forced to subsist on hot sauce packets and sugar in the raw from starbucks.
i, however, have bigger fish to fry than the city's ineptitude at dealing with snow (though i know i'm not the only one banking [har har har] on a snow day tomorrow). i am in the final stages of test driving recipes for the thanksgiving potluck. i thought i had it all figured out, including the pie i wanted to make, until a sudden stroke of inspiration hit me this morning and threw a wrench in my dessert plans. more on this once i've worked out the details.

so this is my newest brussels sprouts experiment. a prior batch, above, was roasted in olive oil and a little bit of honey and then tossed in sesame seeds once out of the oven, and they were decent, especially the hyper-caramelized bits that tasted like candy, but they weren't blowing my mind.
so i've reworked this recipe from the new york times for brussels sprouts and chestnuts in brown butter sauce. i was way into the idea of those things together, but had several gripes: i think boiling is one of the worst ways to cook brussels sprouts, the velouté proportions were way out of whack, the dish was leaning towards blandness, and fried shallots made for a fairly colorless presentation. my remedial version includes oven-roasted brussels sprouts, a tweaked sauce, walnuts instead of chestnuts (though either would be delicious), and red onions instead of shallots. i've also de-fussed the recipe a bit, as it was a little involved for my tastes.
when seasoned properly, this makes for a luxurious side dish, with a touch of sweetness from the softened onions. walnuts and browned butter play off of each other's toasty, nutty flavors and rather than turning to diluted sog in a pot of boiling water, brussels sprouts are deep and tactful from a quick dalliance in the oven.
brussels sprouts & chestnuts in brown butter sauce
serves 8 as a side

• sea salt and freshly ground black pepper, to taste
  
• 2 pounds brussels sprouts, trimmed and halved
• 2 tbsp olive oil
  
• 5 tbsp unsalted butter
• 1 large red or purple union, french cut*
  
• 3 tbsp flour
• 1 1/2 cups hot vegetable stock**
• generous pinch cayenne pepper
  
• 2 tbsp lemon juice
• 1 tbsp spicy mustard, optional
  
• 1 cup roasted, peeled chestnuts, halved if large, or use toasted walnuts

*in strips, like this. ignore my photos.
**i used a kind of mediocre bouillon the first time around and regretted it. the flavor of your stock is going to come through a lot, so use a good one--homemade, if possible.

preheat oven to 400°F. toss brussels sprouts with olive oil, salt and pepper to taste (don't be shy) and turn out onto a baking sheet. roast for 15-20 minutes or until softened and beginning to turn golden and crisp, tossing once midway through. remove and set aside, leaving oven on.

melt butter in a 3-quart saucepan. add onion and cook over medium heat, stirring, until softened, 5-10 minutes. use tongs to remove onions from pan, shaking off as much excess butter as possible and reserving it in the pan. place onions on paper towel to drain.
place saucepan with butter over medium heat and cook until butter has a nutty aroma and is turning brown (i aimed on the slightly deeper end of the brown butter spectrum). whisk in flour and cook until mixture is light brown. whisk in stock and cook until sauce has thickened (should take less than 5 minutes). add cayenne, lemon juice, mustard, and salt and pepper to taste (be very generous). add chestnuts and brussels sprouts, folding ingredients together.
transfer to an 8-cup baking dish. scatter onions on top and sprinkle with additional salt (if you have it, a flaky finishing salt is nice here) and pepper. bake about 15 minutes and serve.