In a small pot, cook pasta according to package directions in salted water.
Over medium heat, add olive oil to skillet and heat until shimmering.
Add onion and red pepper flakes and cook for 5-7 minutes, or until translucent. Stir occasionally.
Add garlic and cook another minute.
Add a tin of sardines with oil and break down the fish with a wooden spoon or silicon spatula. Cook until liquid thickens.
Remove from heat and stir in lemon juice.
Drain pasta and combine it with the sardine mixture.
Stir in cheese and parsley. Add more cheese and lemon juice to taste.