It's wintertime and lettuce is pretty expensive right now. Besides being twice the price they cost in summer, the heads of batavia (green leaf lettuce), laitue (Boston lettuce), and feuille de chêne (oak-leaf lettuce) are really small.
So scarole (escarole) and frisée (curly endive) are better choices, even though they cost significantly more per head. Right now, you get a lot more salads out of a head of scarole than you do out of a head of laitue.
And I love escarole and curly endive. They make good salads with other winter foods: beets (of course), garlicky dressings, lardons fumés, and... duck gizzards (gésiers de canard). If you've never eaten duck (or chicken) gizzards in France, I encourage you to try them. They are commonly served in salads in cafés and restaurants all around the country.
Other winter salad ingredients are beans and potatoes. Both are really good in a vinaigrette dressing with a lot of Dijon mustard in it.
Escarole is a salad green that benefits from being tossed in vinaigrette 15 or more minutes before you serve it. That way, it "cooks" in the dressing and the leaves get more tender.
If you want to start with fresh gizzards, you need to let them cook slow and long in fat — preferably duck fat, but you can also do them in vegetable oil. They take a long time to tenderize, but once they do they have great flavor and texture. These are called gésiers confits in French.
The same kind of salad would be good with chicken livers, pork, or poached or sautéed chicken or turkey. Also eggs.