Simple is best I'm finding. Last night I just knocked up a sauce containing:

Tin of chopped tomatoes + extra half tin of water
Good pinch of oregano
3x squashed garlic cloves
Good grind of pepper and a good pinch of sea salt
A marinated big piece of roasted red pepper out of the jar, finely chopped
Good glug of extra virgin olive oil

Threw it into a pan, covered for 15 minutes on a rolling simmer, then took the lid off, reduced it down and then hit it with a hand blender, and then stirred some pasta into it. Done.

I couldn't be arsed to sweat onions down, so got the extra depth from the roasted pepper. Worked well as a lazy but yummy and healthy dish. If I had any anchovies to hand I would have thrown one of those in too, you can't taste it, but you can taste the depth it adds.