Heat olive oil in a cast iron pan, and cook onion and rosemary in oil until onion is soft.
Toss bread cubes with olive oil and onions. Exact measurement of bread is not necessary. Place in a well oiled, deep sided, 9 inch square pan. Toss with 1/2 cheese, and sprinkle remaining cheese over the top.
Beat together milk, cream, and eggs. Pour over bread in pan. It is fine that the bread sticks out of the custard.
Bake for 1 hour, or until browned and a knife inserted in the middle comes out clean (except for melted cheese).