Menu of post based on first letter of title

Hello,
I’m trying to have all posts grouped by the first letter of their title, in a sidebar, with permalink.
If you check my site (in french, sorry), you’ll find in the right sidebar a list of all posts, sorted alphabetically. This list is becoming quite long and I ‘d like to reduce it by grouping it by letter. For example:
a-c
d-f
g-i
…
Clicking on the group would expand and show the posts.
Is there a plugin or widget that allows this? Or do I need to code this myself? (I’m far from an expert ;(

Here is the link to the site: http://www.vindjeu.eu

Any hint on where to start searching would help.

Many thanks in advance.

Phil.