Spiralize the zucchini using the small blade. Use kitchen shears to cut the spirals so they're easier to eat, and lay them out in a single layer on some paper towels. Let the zoodles release their excess water for 30+ mins.
While the zoodles are sweating, chop the tomatoes, onion, garlic, and basil.
Add the oil to a large skillet, and heat it over medium heat.
Once the oil is hot, add the onion and sauté for about 2 minutes - not quite to translucent.
Add the zucchini noodles and cook until al dente. About 5-10 minutes. Stir occasionally so the onions don't burn.
Add the tomatoes and garlic and cook for 2 or 3 minutes.
Add the basil and cook for 1 minute
Make sure to get all the excess water out of the zucchini noodles BEFORE you cook them. To do this, spiralize the zucchini noodles using a spiralizer, and lay them out in a single on a paper towel. Let them sit for 20-30 minutes. During this time they'll release their excess water, and the paper towels will soak it up. With a clean dish towel, or more paper towels, gently squeeze the noodles to get any excess water out. The finished product will be very watery if you skip this step! Use kitchen shears to cut the noodles so they're not so long. This will make it easier to eat. Have all the ingredients chopped and ready to go BEFORE you start cooking them. The onions, tomatoes, garlic, and noodles don't take long to cook. If you throw in your onion and then go to chop all your tomatoes, the onion will be over cooked by the time you're done chopping! It's so much easier to chop everything ahead of time so you can throw it into the skillet when the time comes. If you tolerate dairy, some fresh grated parmesan cheese would be delicious on top!