Heat the oven to 425 degrees. Place the cauliflower on a sheet pan. Drizzle with 2 to 3 tablespoons olive oil, sprinkle with 3/4 teaspoon salt and 1/2 teaspoon pepper, and toss to coat. Roast, tossing occasionally, until tender and browned all over, 20 to 25 minutes. While the cauliflower roasts, bring a large pot of water to a boil.
Heat a large (12-inch) skillet over medium-high. Add the bacon and cook, stirring occasionally, until crisp, 5 to 7 minutes. Using a slotted spoon, transfer the bacon to a paper towel-lined plate. Add the sage leaves to pan and fry until they begin to brown, 1 to 2 minutes. Using a slotted spoon, transfer the sage leaves to the plate with the bacon. Drain all but 3 tablespoons of fat from the skillet (or add additional olive oil, if needed) and remove from heat.
Add 1 tablespoon salt to the boiling water and cook the pasta until just al dente, reserving 1 1/2 cups of the pasta water. Drain the pasta and add to the reserved skillet; heat over low. Pour or squeeze the lemon juice over the pasta, along with 1 cup pasta water and the 1/2 cup grated Parmesan. Toss until the sauce comes together.
Stir in the cauliflower, bacon and sage, adding a splash more pasta water if the pasta seems dry. Season with salt and pepper. Sprinkle with lemon zest, and serve with extra Parmesan on the side.