When I make French Onion soup
I slice my onions very thin.
It also helps to use a small onion so the pieces aren't so wide.
For the cheese I use gruyere cheese.
It should be grated.
In order for the cheese to cook properly you need to have some stale french bread.
It should be toasted lightly on both sides and then sliced fairly thin.
Then you place the bread on top of the soup and then the grated cheese on top of that.
Next put the soup in the broiler so that it melts the cheese.
cooking.stackexchange.com/users/61/Bryant
In the classic recipe the onions are chopped, personally, I like mine very thinly sliced.
The cheese typically used is the French Comté but a good substitute is Swiss gruyère or even Swiss Emmenthal.
cooking.stackexchange.com/users/115/Pulse
I prefer chopped onions, so you know you're eating onions.
The onions, if cooked long enough will be quite tender.
A smaller cut, say diced, won't hold up well once the stock is added; you'll get a pulpy soup rather than onions in stock.
For the cheese, any melting cheese works.
I prefer a medium cheddar, grated over the top and melted under the broiler.
cooking.stackexchange.com/users/110/ageektrapped
I sometimes chop my onions, and sometimes slice.
If I slice them, I slice them in thin wedges, pole to pole.
As for cheese, I use a good Gruyere.
cooking.stackexchange.com/users/2239/Juju
I've had a very good response to a "french onion" style soup where I've used a combination of leeks (yes, leeks!), red onions, white onions and shallots, all chopped to about 1-2mm in thickness.
cooking.stackexchange.com/users/1266/Rob
What is your preferred type of cut for the onion in onion soup?  And how do you put the cheese? A deep chunk submerged in each plate, on top? What kind of cheese do you like?
Onion soup: How to chop the onion and what kind of cheese to put in it?
cooking.stackexchange.com
<cheese><knife-skills><onions><soup>
cooking.stackexchange.com/questions/336
cooking.stackexchange.com/users/89/Vinko Vrsalovic
The onions are traditionally chopped, but sliced is also popular. Any melting cheese is good, but the traditional French Comté or a good substitute like Swiss Gruyère or Emmenthal are preferred. The onions should be thinly sliced and use a good melting cheese like a gruyère.
The onions should be thinly sliced. The cheese should be a melting type, the classic ones are comté and gruyère.