In a medium size bowl, using an electric hand mixer or food blender, blend the eggs and cheese together. Add the heavy cream, sour cream, salt and cayenne pepper. Combine until smooth and creamy. Set aside while preparing the mushrooms.

Heat a large skillet set over medium-high heat and when hot, add the olive oil and the mushrooms. Sprinkle with salt and pepper and cook, stirring occasionally, until the mushrooms have released their liquid and are golden brown, approximately 10 to 15 minutes.

Reduce heat to medium low and, using a spatula, push the mushrooms to the edges of the skillet and add the shallots and garlic to the center of the skillet. Cook for 3 or 4 minutes or until the shallots have become translucent, approximately 3 to 4 minutes.

Remove from the heat and stir in the thyme leaves. Set aside to cool while preparing the baking rings.

For the Double-Crust Pastry

Add the flour and salt to the bowl of a food processor and pulse several times to combine.

Cut the butter into small pieces, or grate on a box-grater, add to the flour and pulse for 30 seconds or until only small pea-size bits of butter remain. Sprinkle the mixture with 6 or 7 tablespoons of ice water and pulse the processor for about 30 seconds or until the flour mixture comes together in a mass. Do not over-process, any remaining crumbs in the processor bowl can be worked into the dough on the work surface.

Transfer the dough to a work surface that has been dusted with flour. Shape the dough (incorporating any of the remaining flour mixture crumbs) into a ball. Divide the pastry ball into two sections, one a little larger than the other and flatten each into a disk.

Preheat the oven to 325°F.  Coat 8, 3 1/8-inch tart baking rings with oil spray and place on a parchment-lined baking sheet.

Roll the dough 1/8" thick. Cut 8, 6-inch circles, and fit the dough into the rings, trimming the top even with the edges.

Spoon 2 tablespoons of the custard into the bottom of each tartlet then add 1/4 cup of the mushrooms. Pour enough of the custard to cover the mushrooms.

Bake the tartlets for 25-30 minutes, or until they are puffed and the tops are just golden.

Remove from the oven and allow to cool on the baking sheet. 

To serve: Using a metal spatula, lift each tartlet off the baking sheet and set on an individual serving plate. Lift the ring up and off the tartlet. If necessary, run a thin-bladed knife around the inside edge of the baking ring to loosen, then lift the ring. 

Garnish the top of each tartlet with a sprig of fresh thyme.