Heat oven to 400 degrees. On a large rimmed baking sheet, toss the Brussels sprouts, onions, oil , 1/2 t. salt and 1/4 t. pepper. Turn the Brussels sprouts cut-side down and roast until golden brown and tender, 20-25 min; transfer to platter.
Five minutes before the Brussels sprouts are finished, melt the butter in a medium skillet over medium heat until foaming. Add the almond and cook, stirring, until the almonds and butter are golden brown, 2-3 minutes. Add the raisins, lemon juice and 1/4 teaspoons salt and swirl the pan to combine. Spoon the brown butter mixture over the sprouts and onions.