What could be added as a modification to dishes without any charge?

Sliced onions and tomatoes can be provided as modifications to dishes without any additional charge because they are commonly considered standard garnishes or accompaniments that enhance the flavor and presentation of a meal. In many dining settings, these items are readily available and do not require extra preparation or significant costs, allowing establishments to offer them at no additional expense to the customer.

On the other hand, options such as sautéed vegetables, extra sauce, and sourdough bread typically involve more preparation and resources that may incur costs, leading restaurants to charge extra for these items as they can significantly add to the dish's volume and overall quality. This distinction helps set expectations for customers regarding what modifications can be made without affecting the final bill.