Preheat the oven to 425 degrees F. Line a rimmed baking sheet with foil. Toss the Brussels sprouts, oil and a generous pinch of salt and pepper on the baking sheet until coated. Spread the sprouts, cut-side down, in a single layer and roast until tender and browned, 20 to 25 minutes.
Meanwhile, heat the butter in a medium skillet over medium heat until it starts to brown. Add the garlic and cook until soft, about 30 seconds. Add the breadcrumbs, thyme, pepper flakes and a pinch of salt and cook, stirring occasionally, until the crumbs are golden brown, 2 to 3 minutes. Remove from the heat and stir in the lemon zest and parsley. Scatter over the Brussels sprouts. Garnish with thyme sprigs if using.
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