Question

Do you ever use “a”/an in front of food when you order it in a restaurant?
I hope you can have a look at the following orders and tell me whether or not they sound better with the indefinite article. Thank you.

I'll have ...
pasta with clams and mussels/tomato sauce/ meat sauce/ butter and sage/gravy
grilled/fried/roast chicken breast (roast leg of lamb, a grilled sirloin steak, a hamburger, a breaded cutlet, rabbit stew, braised beef etc..)
a poached/fried/soft-boiled/hard-boiled egg
cod/plaice/fried sole with spinach/carrots /courgettes/ cauliflowers/ asparagus/ spinach/ fennels/ peas/ roast/mashed/boiled potatoes/chips/ beetroots/ red cabbage/ Brussels sprouts/ French beans/ beans ….
a Half-a-litre bottle of fizzy water
some grapefruit juice.

Answer 1

The article "a" or "an" is used when you are specifically referring to ONE thing:

a piece of steak

a cup of coffee

a hard-boiled egg

But when referring to something you cannot count (such as coffee, cereal, pasta), then the article is not used.

Basically, you're dealing with "count and non-count nouns" --
http://leo.stcloudstate.edu/grammar/countnon.html
and
http://grammar.ccc.commnet.edu/grammar/noncount.htm

Let me know if you have more questions.