For the tartlet pastry: Add flour, butter and salt to the bowl of a food processor and whirl until the mixture resembles coarse crumbs. Add cream cheese and egg and pulse until combined and the dough starts to clump together. Divide dough into 2 portions and shape each into a large 6” to 7” disk. Wrap each one tightly in plastic wrap and refrigerate for 1 – 2 hours until firm.

Meanwhile, make the filling. In a large 12” skillet heat 1 tablespoon of the oil over medium heat. Add onions and sauté for 8 minutes while stirring occasionally, until lightly golden and caramelized. Raise heat to medium-high, add remaining 1 tablespoon oil and mushrooms; continue to sauté until the mushrooms are browned and tender, about 6 - 8 minutes.

Add red peppers and sauté until the peppers are just tender, about 2 minutes. Add balsamic vinegar and stir to combine until the all liquid evaporates. Remove from heat, season with salt, pepper and fresh thyme. Set the mixture aside to cool slightly. Set aside the goat cheese until ready to fill the tartlets.

Roll out the tartlet pastry dough: roll each disc of dough on a lightly floured work surface into a 14” circle (about ⅛” to ¼” thick). Cut out 48 three-inch circles, re-rolling the scraps as necessary. Place the circles into the cavities of the muffin pan by gently pressing them into the bottom and up the sides, bringing the edges of the dough to just above the rims. You may have some extra dough remaining, depending on how thick the rounds were cut. Refrigerate tartlet shells for 10 – 20 minutes to chill.

Meanwhile, preheat oven to 375 degrees F.

While the oven is preheating and the pastry shells are chilling, make the egg custard. In a large bowl, whisk together eggs, cream, mustard, salt, pepper and nutmeg.

Evenly drop a dollop of goat cheese into each tartlet cup. Top each with 1 rounded teaspoon of the onion- mushroom filling. Gently pour in 1 tablespoon of the egg custard over the filling in each tartlet, filling it just to the rim. Do not overfill as the egg custard will puff up while baking.

Immediately bake for 18 – 20 minutes until lightly golden and crispy and the egg custard has set and puffed up. Remove from oven and let cool in pan for 5 minutes. Remove tarts to a cooling rack. Serve warm or at room temperature garnished with more fresh thyme.

Note: If making tartlets ahead of time, reduce baking time by 2 to 3 minutes. Cool tarts completely and store in an air-tight container in the refrigerator for up to 3 days or in the freezer up to 3 months. To reheat, place cold or frozen tartlets on a baking sheet (or back into the mini muffin cups) and heat in a preheated 350 degrees F oven for 10 to 15 minutes until lightly golden and warm.