Many foodstuffs are considered uncountable when prepared and countable in the field or on the farm: I like apples, chickens, and tomatoes on the farm. I like apple pie (baked apples still look like an apple), roast chicken, and pizza with tomato and pepperoni in the restaurant. Pizza, spaghetti, stew, ham and such dishes don't look much like any countable object anymore, so are considered in the uncountable form.
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