In a saucepan, gently cook the onion and garlic in the olive oil until the onion has softened. Add the crushed tomatoes, red pepper flakes and sugar.
Season with salt and pepper and simmer for 25-30 minutes until thickened.
Meanwhile, cook the pasta in plenty of salted water until cooked 'al dente', and then drain the pasta. Reserve 1/2 cup water in the sauce needs to be thinned.
Add the pasta to the sauce and mix in well.
Sprinkle with grated parmesan and serve immediately.