Wash and dry squash then cut them into thin rounds (around ¼" thick). Slice onion. Heat pan over medium heat. Add olive oil and onions. Cook until they begin to soften.
Add squash and spread out into an even layer. Continue to cook for 2-3 minutes. Season with salt and pepper and stir. Add butter and water and cover.
Let the squash steam for 7-10 minutes or until the water has evaporated. Remove the cover and stir. Taste and add salt and pepper, if necessary.