Which sauce is made with onions and mustard to create a brown sauce?

The sauce made with onions and mustard to create a brown sauce is indeed Robert Sauce. This sauce typically includes ingredients such as onions, white wine, and mustard, which contribute to its distinctive flavor profile that complements meats and other savory dishes. Robert Sauce is a classic French sauce that falls under the category of brown sauces, which are often based on a brown stock and thickened with a roux.

In contrast, the other options listed do not align with the criteria of being made with onions and mustard. Roux is a thickening agent made from equal parts flour and fat, commonly used in sauces but not a sauce itself. Alfredo Sauce is a creamy sauce made with butter, cream, and cheese, primarily associated with pasta dishes. Marinara Sauce is a tomato-based sauce that typically includes garlic, onions, and herbs but does not include mustard or the elements that define a brown sauce. Thus, Robert Sauce stands out as the correct answer for this question.