Preheat oven to 400°F (200°C). Toss sliced fennel with 2 tablespoons olive oil, salt, and pepper. Spread on a baking sheet in a single layer. Roast for 25–30 minutes, stirring halfway, until caramelized and tender.

Meanwhile, bring a large pot of salted water to a boil. Cook pasta according to package instructions until al dente. Reserve 1/4 cup pasta water, then drain.

In a large skillet, heat remaining 1 tablespoon olive oil over medium heat. Add garlic and red pepper flakes (if using) and sauté for 1 minute until fragrant.

Add roasted fennel and cooked pasta to the skillet. Toss to combine, adding reserved pasta water as needed to loosen the sauce.

Remove from heat and stir in Parmesan cheese and lemon zest. Taste and adjust seasoning with salt and pepper.

Serve warm, garnished with chopped parsley if desired.