Trillium albidum

From Oregon Flora, Gardening with Natives:

Giant trillium is a Northwest forest favorite for its fragrant sessile white flowers which bloom from March to June. Mottled broadly oval leaves subtend creamy white petals and lanceolate sepals, all arranged in characteristic tripartite symmetry. An understory plant of moist forests, it does well in dappled light of woodland gardens or at the edges of water features. Slow growing, it can spread through seed and rhizome to form a lovely groundcover. Protection from slugs may be needed.