Bring a large pot of salted water to a boil.
Cook the spaghetti until al dente. Reserve 3/4 cup pasta water, then drain.
Whisk ricotta, Parmesan, lemon juice, lemon zest, 1 tbsp olive oil, salt, pepper, and red pepper flakes in a bowl until smooth.
Warm remaining olive oil and butter in a large skillet over medium heat. Gently sizzle the garlic for 30 seconds.
Add drained pasta to the skillet. Toss in the ricotta mixture and a splash of pasta water.
Stir briskly until the sauce turns silky and coats the pasta, adding water as needed.
Fold in herbs. Taste and adjust salt, pepper, or lemon to your liking.
Serve hot with extra Parmesan and a pinch of zest if desired.