The sprouts sear cut-side down in bacon fat before anything else touches the pan, and the half-and-half only ever simmers gently, so the parmesan melts into it instead of clumping.

Bring a large pot of water to a boil and salt it well.
Put the bacon in a large, cold skillet and set it over medium heat — starting cold lets the fat render out before the meat browns. Cook 8 to 10 minutes, until the bacon is crisp and the pan is slick. Lift the bacon onto a plate, leave about 2 tablespoons of fat in the pan and pour off the rest.
Turn the heat to medium-high. Lay the sprout halves cut-side down in a single layer and leave them alone for 4 to 5 minutes, until the undersides are deep brown; if they're only golden, give them another minute. Toss and cook 2 to 3 minutes more, until a knife slides into the thickest one with little resistance. Add the loose leaves for the last minute so they wilt and crisp at the edges, and season with salt.
Meanwhile, cook the spaghetti 1 minute short of the package time. Before draining, scoop out the pasta water you need, plus a little extra.
Turn the skillet down to medium-low. Add the shallot, garlic and pepper flakes and stir 1 to 2 minutes, until the garlic smells sweet but hasn't browned. Pour in the half-and-half and the measured pasta water and hold it at a gentle simmer — small bubbles at the edge, never a rolling boil, which can split it — for about 2 minutes, until it lightly coats a spoon.
Add the drained spaghetti and toss for 1 to 2 minutes, until the sauce clings. Take the pan off the heat. Add the parmesan a handful at a time, tossing between each, then the lemon zest, a squeeze of juice and the bacon. Keeping the lemon off the heat matters: acid near a boil is what curdles dairy. Loosen with more pasta water a splash at a time — the sauce should pool slightly in the pan, because it tightens as it sits.
Serve right away with more parmesan and plenty of black pepper.
Made it? Be one of the first to rate it.