Delicious French Tarte a l’Oignon

Directions

1. 1

   Caramelize the onions slowly - don't rush this

   Melt the butter in a large skillet over medium-low heat, add the sliced onions and sugar, and cook, stirring occasionally, for a full 30-35 minutes. The onions need this long, unhurried time to break down and turn a deep golden brown - trying to speed this up with higher heat will just make them burn before they sweeten.

2. 2

   Let the onions cool slightly

   Take the caramelized onions off the heat and let them cool for a few minutes before combining with the eggs, so they don't start cooking the custard prematurely.

3. 3

   Whisk a smooth custard base

   Whisk together the eggs, heavy cream, and nutmeg in a bowl until fully smooth and evenly combined.

4. 4

   Layer the tart

   Spread the cooled caramelized onions evenly across the blind-baked crust, scatter the grated gruyere on top, then pour the custard mixture over everything, letting it settle into all the gaps.

5. 5

   Bake until just set

   Bake at 375F (190C) for 30-35 minutes, until the custard has puffed slightly and turned lightly golden, with only a slight jiggle remaining in the very center.

6. 6

   Rest before slicing

   Let the tart cool for about 10 minutes before cutting - this short rest allows the custard to finish setting so your slices come out clean rather than runny. Serve warm or at room temperature.