Matilija Poppy (Romneya coulteri). Large herbaceous perennial.
Give these full sun, lots of room and let them go. They’ll happily spread all over your garden, but when planted at the back where their flowers appear above other plants this won’t be a problem – as long as they have space to thrive. They need good drainage, so plant on mounds, slopes or in well-draining soil.
You can cut the plants back in summer when they’re dormant to improve appearance. Other than that, they require little or no care (or water).