Prepare a deliciously creamy pasta with garlicky Brussels sprouts in just one pot. Start by boiling a large pot of water and generously seasoning it with salt. Add spaghetti and cook for 2-3 minutes less than the package instructions. Meanwhile, heat oil and garlic in a large skillet over medium-low heat. This method prevents the garlic from burning. Cook until sizzling, approximately 1-2 minutes. Optionally, add a pinch of crushed red pepper for some heat. Stir in Brussels sprouts and increase the heat to medium-high. Cook, stirring often, until the sprouts are wilted and tender, about 5 minutes. Squeeze the juice of half a lemon into the skillet and stir. Season with salt and pepper. Add ¾ cup of pasta water and bring to a simmer. Use tongs to add the spaghetti to the skillet and toss to combine. Stir in butter until melted. Turn off the heat and gradually stir in cheese, tossing constantly with the tongs to evenly coat the pasta.