Casserole and Closed Mussels is one of a group of early assembled works by Marcel Broodthaers that use mussel shells as both their subject and medium. It was made in 1965, only a year after Broodthaers embarked on a career as a visual artist after several years as a poet, and before he gained the international success that would follow. As much from financial necessity as anything else, he used discarded everyday objects, including mussel and eggshells, second-hand pots, pans, and glass jars, to create these early sculptures. The mussel shells in this work were obtained from a restaurant that he frequented in Brussels. They were then cleaned by his wife, Maria Gilissen. They rise up in a column out of a black painted iron casserole with wood and brass handles. The enameled lid sits on top of the mussels. The cooking pot belonged to the Broodthaers’s family and was used right up until the time that the work was created, lending it a personal and biographical element that is a feature of many of his early sculptural pieces and later museum projects alike.

Broodthaers experimented with various arrangements for the shells before deciding on the present configuration. He enclosed the mussel shells in a bag to hold them in place and poured liquid polyester resin over them to retain the shape permanently once it hardened and the bag was removed. The resin contains green pigment, which remains visible on the mussel shells and, according to the artist, reminded him of the sea. Broodthaers produced several other casserole and mussel works during 1965–66 such as Triumph of the Mussels II or Red Mussels in a Cooking Pot. He also combined mussels with pieces of furniture, canvases, and an open suitcase. However, Casserole and Closed Mussels is the only work in which he used closed rather than open mussels. The significance of this may be to do with the fact that closed mussels are inedible: to be eaten they should open during cooking. Casserole with Closed Mussels therefore presents the viewer with an inherent contradiction. Broodthaers was highly aware of the aesthetic appearance of his work, and its potential to go beyond the everyday circumstances of the materials he used. He stated of his mussel works: ‘A mussel conceals a volume. When the mussels overflow the pot, they are not boiling over in accord with physical law, but following the rules of artifice whose purpose is the construction of an abstract shape.’

The choice of mussels was not merely an aesthetic one for Broodthaers. The mussel shell can read as symbolizing the artist’s native Belgium, mussels and chips being a popular national dish of that country. Broodthaers’s use of mussels may also make reference to the traditional representation of food, in particular shellfish, in Flemish still life painting. Here, particularly during the seventeenth century (like in Osias Beert's Still Life with Oysters), the empty shell became a regular symbol of vanity and the futility of luxury. In Casserole and Closed Mussels this is experienced all the more strongly for the fact that the closed mussels cannot be eaten. As such, Broodthaers may also have intended the mussel to serve as a somewhat satirical symbol of the bourgeoisie.