There are not so many differences in the meaning between 생각한 and 생각하던 in that context . 생각한 sounds more common and usual for me and 생각하던 sounds more like 'have been thinking' or 'was thinking' for a specific time duration? It's really hard to explain but 생각하던 sounds more like someone keeps an idea for a while not like short moment while 생각한 just simply means "thinking " so "the food I was thinking of is pizza"= 내가 생각한(생각하고있던) 음식은 피자야” sounds just smiliar I think. and " the food I thought of was pizza =내가 생각했던 음식은 피자야