Heat oil in a large skillet over medium-low heat. Add onion and cook until soft. Add tomatoes and simmer until tomatoes burst. Add garlic and cook until fragrant. Add squash and zucchini and cook, gently stirring, until crisp-tender. Season with salt and pepper.
NOTE: During April and May we always have a ton of yellow squash and zucchini. This is what I do with most of it.