Oven temperature: 160° C (325° F)
Preparation time: 30 minutes
Refrigeration time for the pastry: 4 hours
Cooking time: Under an hour

Making the pastry 

1. Make the pastry 4 hours in advance so that it firms up and loses some of its elasticity;
2. rub the butter and flour together until crumbly;
3. in a bowl, combine the other ingredients; gradually blend into the first butter/flour mixture;
4. roll into a ball; cover with plastic wrap and refrigerate. 
5. Roll out the pastry on a lightly floured surface.
6. Line 4 well-buttered brioche molds with pastry. Refrigerate until completely chilled or until ready to use. 
7. Bake in a 160°C. (325°F.) oven for 15-20 minutes.
8. Remove from the oven; cool and unmold.

Confit onions

1. Gently cook the onions in butter with a pinch of salt; drain in a strainer to remove the excess liquid and butter;
2. Sauté the bacon bits in a few drops of oil until browned; drain; 
3. Pour the cream into the same skillet and reduce by half; add the onions and bacon; season with salt and pepper.

Preparing the quails and finishing

1. Roast the quails in a 200°C. (400°F.) oven for 6 minutes; let rest for 10 minutes; cut in half; debone the breast but leave the bone in the leg.
2. Poach the quail eggs by breaking them into boiling water to which a little vinegar has been added;
3. Remove with a strainer once the white begins to coagulate; refresh in cold water and set aside. Just before serving, reheat briefly by immersing them in boiling water for a few seconds with a strainer. 
4. Cook the spinach and drain. Divide among the tartlets. 
5. Sauté the mushrooms and summer truffles; season with salt and pepper; place on the spinach. 
6. Add a spoonful of confit onions and cover with 4 quail eggs; 
7. Garnish with boned quail and serve immediately.