The filling can be either chicken, beef, shrimp or tuna, and yes there are many different variations on how you make the filling, I think it's all up to individual taste.
This is how I make the tuna filling.
Rissois de Atum
Dough for the rissoles
2 cans of tuna, drained
1 teaspoon olive oil
1 small onion, finely chopped
salt and pepper
In a medium saucepan, add the onion and the olive oil, saute for a few minutes until translucent. Add in the tuna and season with paprika, salt and pepper to taste. (At this point, you can add some tomato paste if you wish, but for this one I didn't because the paprika was enough).
Mix well, then sprinkle in some flour. I don't have an exact measurement, I eyeball it, but I would say about a tablespoon, just enough to thicken it. Pour in some heavy creamy and stir well until creamy and smooth. If you want it a little thicker then add in a little more flour, and if it's too thick, then add in a little more cream.
Proceed to make the rissoles as shown in the link above.