1 large onion
2 cloves garlic
1 teaspoonful tomato puree (tomato paste)
1 400g (14 oz) tin chopped tomatoes
6 oz water, stock or wine
Seasoning

Chop onion and garlic, microwave for five minutes (omit this step if
you don't have a microwave; it isn't essential but it makes the
sauce quicker to cook).  Transfer to saucepan, add tomato puree (paste) 
and stir.  Add tinned tomatoes and other fluid (I use half the empty
tin as a measure, which has the added advantage of rinsing it out!).
Season, bring to boil, and simmer until the pasta is cooked (which
you are meanwhile cooking in another pan).  For seasoning, I use
salt, freshly milled black pepper, Worcestershire sauce and some sort
of herbs; fresh basil if I have it, or dried Italian seasoning.  But
use what you like and have handy.

I add frozen peas and sweetcorn to this, to make a balanced protein,
perhaps mushrooms, chopped peppers, etc.  You can add bacon (back,
all visible fat removed, of course :-) ) at the tomato puree stage,
and season with chilli for pasta amatriciana.  Or sometimes I omit
the tomato puree, use white wine for the liquid, and use as a
sauce for microwaved white fish, served on brown rice.  The possibilities
are endless.