This is soup. This is a bowl of soup; would your friends like some soup? This is cheese. Would you like some cheese? I could never eat a pound of cheese! This is butter. I love to eat butter on my bread. This is a head of lettuce. I like lettuce. I eat lettuce fairly often. This is a cake or this is cake [either is correct, but saying 'a cake' implies the whole cake is there]. I'm having dessert; would you like some cake? Would your friend like some of this cake, too? This is an onion. It makes me cry when I cut it. This is beer or this is a beer? [either is correct, but note the use: Would you like a beer? I have a bottle of beer for you. Do you like beer? I'm really thirsty; where's the beer?] At the bar, you can order one beer, or two beers, or many beers. I'd like to order 6 beers for that table over there where my friends are sitting.

soup U cheese U butter U lettuce U cake C onion C/U beer U

Some nouns can be both countable and uncountable, depending on context. Most of your list fall into that category. Soup is usually uncountable. You can sometimes use it as countable if you were referring to a recipe, or a dish, and usually in abstract terms. 'Would you like a soup to start?'. 'I sometimes make a vegetable soup in the winter'. If it is a block of cheese, it is a cheese. If it is a smaller quantity of cheese, it is cheese. Butter is uncountable. We would usually say 'a pound of butter' or 'a pack of butter'. Lettuce is countable if it is an entire head of lettuce. It is uncountable otherwise (e.g. there is some lettuce in this salad) The same is true with cake and onion. If you have a whole one, it is 'a'. If it is cut into pieces, it becomes some. A beer is a bottle or a glass of beer. i.e. a serving of beer. The actual liquid is uncountable. If you pour some beer into a glass, you have poured a beer.

This may help: https://en.learniv.com/info/en/grammar/countable-and-uncountable-nouns/