"Maroilles" is a proper noun, the name of the cheese and of the village where the cheese is made, and a quick online search confirms it is 'Tarte au Maroilles', as expected. There does seem to be an exception in the heading of one English-language site (enough said). 

The reference is to the use of "Maroilles" cheese generally, in an uncountable sense (although recipes will 'quantify' as for example, 250 grams etc, but not a 'number of Maroilles cheeses' 

https://fr.wikipedia.org/wiki/Maroilles_(fromage)

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