Lentil salad with oil and vinegar

I've started making this cold (or room temperature) lentil salad. My wife who isn't much of a lentil fan, likes this. When I made this a week ago, I skipped the mirepoix and just used water. It worked very well. I also prefer french lentils which have a slightly earthier taste and remain firmer than other green or brown lentils. Don't use dahl lentils as I think these will be too mushy.

(Optional) Mirepoix of onions, carrots and celery with olive oil.
2 cups french lentils, rinsed and sorted.
5 cups of liquid, water is fine, or a broth.
Extra virgin olive oil
Vinegar, preferably white or red/white wine or even cider
Salt and pepper

Rinse the lentils and remove anything you don't want to eat, like sand.
Finely chop the mirepoix and fry it in olive oil over med-high heat until mostly soft. Add the lentils and the liquid. Simmer until cooked, but not mushy, about 45 minutes (?) for french lentils and 35 minutes (?) less for other lentils. There should not be much water left. If there is excess liquid, ladle most of it away.

Mix or shake or wisk olive oil, vinegar, salt and pepper to taste. Stir into the lentils while they are still warm. Taste. I use slightly more oil than vinegar, but it is easy to add more of either to the lentils to suit your preference.

Optionally dress this up with cherry tomatoes and/or some goat cheese. Also, I think green olives might go well with this too, but I haven't tried this.