Bring a large pot of salted water to a boil.
Cook the pasta until just al dente (usually a little less time than the package instructions indicate).
Using a serrated knife, carefully saw the baguette, if using, into thin slices. Using your fingers, crumble the bread to create a nice mixture of coarse and fine crumbs.
Heat 2 tablespoons oil in a large skillet over medium heat. Add the crumbs and gently fry them, stirring occasionally and letting them slowly take on color. (You may need to add up to 2 more tablespoons oil to the skillet, depending on just how many bread crumbs you have.)
When the bread crumbs are golden and crisp, add the garlic and fennel seeds, stir, and cook for a minute or so more. Season the crumbs quite generously with salt, pepper, and red pepper flakes. Remove from the heat.
Drain the pasta, reserving the cooking water. You don't need to be obsessive about draining the pasta until no water clings to the strands; in fact you want just a touch of the pasta water to cling to the pasta.
Toss the pasta in the skillet with the bread crumb mixture. Drizzle with oil—preferably quite a lot more oil for the best results. If the mixture seems dry, add a dribble of the pasta cooking water. If using, sprinkle with grated pecorino Romano to taste.