Complex sentence: Lucian Freud was a German-born British painter, known chiefly for his thickly impastoed portrait and figure paintings, who was widely considered the pre-eminent British artist of his time.
Split into simpler sentences: Lucian Freud was a German-born British painter. Known chiefly for his thickly impastoed portrait and figure paintings, he was widely considered the pre-eminent British artist of his time.