Step by step recipe:

1. 5 min.

   Prepare the radish pickle

   Rinse 3 radishes and dice very small (known as a brunoise).
2. 3 min.
   Put the diced radishes in a ramekin, add 4 tablespoons vinegar, salt, pepper and the roughly chopped leaves of 1 sprig tarragon.
   
   This condiment can easily be made the day before and left in the fridge until needed, covered with plastic film.
3. 5 min.

   Prepare the mushrooms

   Peel 200 g mushrooms, choose large ones and break off the stalks (use these in another recipe).
   
   Preheat the oven to 390°F (200°C).
4. 6 min.
   Pour 3 tablespoons olive oil into a non-stick frying pan on high heat. When hot, add the mushroom caps.
   
   Sauté until just browned on both sides, then salt lightly at the end of cooking.
5. 2 min.
   Transfer the sautéed mushrooms to an ovenproof dish.
6. 10 min.
   Prepare 150 g onion, chop finely, then fry in 50 g butter.
7. 1 min.
   Salt and add the chopped parsley at the end of cooking.
8. 3 min.
   Divide the fried onions between the mushroom caps.
9. 3 min.
   Fill with a generous tablespoonful of rillettes.
10. 20 min.
    Bake for about 20 minutes, until the rillettes start to brown.
11. 
    Serve while still hot with a little of the radish pickle on each mushroom.
    
    You can serve these on their own or, as shown here, with cubes of fried polenta.