Heat the olive in a large skillet and then add the onions. Cook at medium heat, for about 2 minutes, stirring infrequently. Stirring infrequently is the key to a great saute - the more your stir, the less the veggies will be crisp as desired. Add the bell pepper, let that cook for a couple minutes and then add the squash. Stir to combine and then leave it alone for a few minutes.
Sometimes I will cover this will a lid for a few minutes, but it depends on how everything is cooking. Check to see how to squash is cooking, flip everything over in the skillet or stir gently. If you are not stirring it often, you will see the edges begin to crisp and brown on the vegetables.
If the skillet is dry or things appear to be sticking to it, just splash a little bit of water into the skillet. When the veggies are done to the level that you desire, remove from the heat and serve immediately. Enjoy!