Hello Nada. Strictly speaking, lamb is the meat of a ...lamb, that is a baby sheep (under one year old), and mutton is the meat of an older sheep... See this: https://www.thespruce.com/the-difference-between-lamb-and-mutton-2356034 . In terms of the grammar, I would say the first sentence using the possessive case, with the apostrophe, although what you wrote is not incorrect per se: 'Lamb is a (young) sheep's meat'. Regarding the second phrase, the word menu comes from the French 'menu de repas'. literally 'list of what is served at a meal' (https://www.etymonline.com/word/menu), so what you wrote is correct.

1. Lamb is the meat of a baby sheep. (Mutton is the meat of an adult sheep) 2. The/A menu is a list of food in a restaurant. (Correct with 'the/a menu') Sheep is one of the rare irregular plurals in English. So, it is both singular and plural. There is a sheep outside. (Just 1) There are many sheep outside. (More than one) Other animals that can be both singular and plural are fish and deer. a fish / lots of fish a deer / lots of deer