Which ingredient is included in both Mornay sauce and Tomato Fondue?

The ingredient that is present in both Mornay sauce and Tomato Fondue is butter.

Mornay sauce is a classic derivative of béchamel sauce, traditionally made from a mixture of butter, flour, and milk, thickened to a creamy consistency. Cheese is then added to this base, typically Gruyère or other similar cheeses, to create the rich flavor profile characteristic of Mornay. Butter serves as the foundational fat, providing richness and a smooth texture to the sauce.

Similarly, in Tomato Fondue, butter is often used to sauté vegetables or incorporate with tomatoes, adding depth and richness to the sauce. While various flavors and ingredients may distinguish Tomato Fondue, the base often relies on butter for its flavor and mouthfeel.

Although cream plays a role in many sauces, including some variations of Mornay, it isn't a primary component of Tomato Fondue. Egg yolks are typically used in sauces that require emulsification, such as hollandaise, but they are not a standard ingredient in either sauce. Cheese, while a key ingredient in Mornay, is not found in Tomato Fondue. Thus, butter is the common ingredient linking both Mornay sauce and Tomato Fondue.