Is it correct to say, for example: "She fed him on lamb" as well as "She fed him with lamb"? Omitting the preposition and simply leaving the sentence as "She fed him lamb" sounds more acceptable to me, but I'm interested in the difference in meaning using either one preposition or the other. Thanks!
"She fed him lamb" is the best way to say that she gave him a meal of lamb. The phrase "She fed him on lamb" implies a steady diet of lamb that she's feeding him. The phrase "She fed him with lamb" doesn't sound as correct as just leaving out the preposition entirely.
How did her dog get so big? She fed him on lamb (habitually).
What did she feed the dog for dinner? She fed him lamb (just once).
The eater "feeds on" something. Polar bears feed on seals.
The person who gives someone (or an animal) food, "feeds" the person (or the animal). She fed the child porridge. He feeds the pigeons (with) breadcrumbs.
In your "fed him on lamb" example, you're conflating these verbs.