Cook penne pasta per package directions in boiling salted water.
Meanwhile, on a large baking sheet, toss Brussels sprouts and garlic with oil, red pepper flakes, and 1/4 teaspoon each salt and pepper. Scatter bacon over the top and roast for 15 minutes. Sprinkle sprouts with 1/4 cup Parmesan and toss to combine, then broil on the top rack until bacon is crisp and Brussels sprouts are golden brown and tender, 1 to 2 minutes.
When pasta is done, drain pasta, and return to pot. Finely grate lemon zest onto pasta, then squeeze in juice and toss to coat. Toss with remaining 1/4 cup Parmesan, broth and butter. Serve Brussels sprout mixture on spaghetti with additional Parmesan, if desired.