Make the sauce in a high-speed blender. Puree the cashews, miso paste, garlic, Dijon mustard, water, lemon juice, olive oil, and a pinch of salt until smooth. Taste and adjust the seasonings.
Heat the oil in a large skillet over medium-high heat. Add the brussels sprouts, cut side down, along with a pinch of salt. Let them sear until the cut side becomes golden brown, 2-3 minutes. Toss and continue cooking for an additional 7-10 minutes, or until tender.
Add a squeeze of lemon juice to the pan, toss once more, then remove from the heat and transfer to a plate.
Bring a large pot of salted water to a boil. Prepare the pasta according to the instructions on the package, cooking until al dente. Reserve 1 cup of the pasta water. Drain and return the pasta to the pot. Stir in the sauce, adding pasta water as needed to thin the sauce and make it creamy. Add the brussels sprouts and toss until heated through.
Serve pasta hot with freshly ground black pepper and chili flakes, if using.