Replacement for pasta that is natural and fairly low carb I'm looking for something with the physical properties of pasta (spaghetti or similar, not lasagne). Something that I can toss with a sauce or oil and serve in a large bowl for a weekday supper. Something grain-free and natural.

I've tried a large onion cut into thin strips (probably the most successful) and leeks (tend to be too floppy / wet).

Any other suggestions?
The classic example would be spaghetti squash.  It has the unique property that when you cook it, it can be pulled apart into long, thin, pasta-like strands.

You can then sauce it or use it in a manner analogous to pasta, but its nutritional profile is far more similar to squash, because, well, it is squash.