Cook the spaghetti according to the package instructions until al dente. Drain and set aside.
In a large skillet, heat the olive oil over medium heat. Add the garlic and cook until fragrant, about 1 minute.
Add the thinly sliced fennel to the skillet and sauté until softened, about 5 to 7 minutes.
Add the red pepper flakes, salt, and black pepper to the fennel and stir to combine.
Add the cooked spaghetti to the skillet, tossing to coat with the fennel mixture.
Remove from heat and stir in the grated Parmesan cheese.
Serve hot, garnished with additional Parmesan and fennel fronds if desired.