Complex sentence: It was Monseigneur René Nicolas Sergent who pressed for an altar to match the grandeur of the cathedral and Mon.Boeswilwad, the architect in chief of "Monuments Historiques" was put in charge of the project and commissioned the goldsmith Placide Poussielgue-Rusand to create the altar.
Split into simpler sentences: It was Monseigneur René Nicolas Sergent who pressed for an altar to match the grandeur of the cathedral and Mon.Boeswilwad, the architect-in-chief of "Monuments Historiques" was put in charge of the project. He commissioned the goldsmith Placide Poussielgue-Rusand to create the altar.