Spain mulls action over blame for E. coli outbreak
Mull, in this case, means 'to consider'.
The OED gives meaning "mull" 4.1.b. trans. U.S. To consider, ponder upon.
I don't recall having come across this transitive use (less surprising if it is a specifically US use) , but I am familiar with meaning 4.1.c: "to mull over : to turn over (an idea, etc.) in one's mind; reflect upon, think over, ponder."