Queen anne's lace is a biennial, meaning they grow leaves the first year, and bloom the next. They are a member of the carrot family, and is considered a
weed in some places (including here in Canada). I don't think it came with your spinach
seeds. More likely the
seed was dropped by an animal or was blown there by the wind. They form carrot like taproots and can be difficult to get rid of.
*IMPORTANT* queen anne's lace looks like another plant called water hemlock, which is VERY toxic. Hemlock has cyanide in it and was used to kill Socrates. Make sure you wash your hands well after touching the plant, just in case.