Bring a large pot of water to a boil. In a small saucepan, warm 3 tbsp olive oil over low heat. Add garlic, chili flakes, salt, and a basil stem. Let gently infuse without browning. Remove from heat and let cool completely.
Salt boiling water and cook pasta until al dente. Once water is starchy, carefully add 1–2 cups to the infused oil (make sure oil is not hot). Cover and simmer 5 minutes, then blend until smooth.
Before draining pasta, reserve 1 cup pasta water. Return pot to low heat. Add mascarpone, blended mixture, lemon zest, and juice. Stir until smooth. Add pasta, fresh basil, and toss continuously, adding reserved pasta water as needed until glossy and silky.
Serve immediately. Finish with black pepper, basil, and grated cheese if desired.
