Bring a large pot of salted water to a boil. Add the halved Brussels sprouts and blanch for 3-4 minutes until just tender but still bright green. Drain well and pat dry.
Heat a large frying pan or skillet over medium-high heat. Add the chorizo slices and cook for 3-4 minutes until crispy and the oils have released. Remove with a slotted spoon and set aside.
In the same pan with the chorizo oil, add the olive oil and heat. Add the Brussels sprouts cut-side down and cook without stirring for 4-5 minutes until they develop a gorgeous golden crust.
Toss the sprouts and add the sliced garlic. Cook for another 2 minutes, stirring occasionally, until the garlic is fragrant and golden.
Add the chopped chestnuts and crispy chorizo back to the pan. Toss everything together and cook for 2-3 minutes until the chestnuts are heated through.
Drizzle with balsamic vinegar and add the butter. Toss until the butter melts and coats everything in a glossy, delicious glaze. Season with salt and pepper to taste.
Transfer to a serving dish and sprinkle with fresh thyme leaves if using. Serve immediately while everything is hot and crispy. Trust me, you'll want to make a double batch!
