This is one of our favorite tarts to serve at Brunch. It's classic French bistro fare. Creamy and savory with a hint of sweetness. It's the perfect combination and disappears within minutes. Serve with a salad of field greens and you have an entire meal. The dough recipe is our standard pie crust recipe. We use it for everything! —Renee's Kitchen Conundrum
Cut butter into very small pieces and add to the flour and salt mixture. Turn the mixer on low and slowly add the water.
When the dough is a sandy texture, remove it from the bowl of the mixer and knead into a ball. Cover in plastic wrap and place in the refrigerator for one half hour. Preheat oven to 400 degrees F.
When ready, remove from the fridge and roll out on a floured surface about 1/4" thick. Butter and flour a 9" tart ring/mold or a 9" shallow pie pan. Place the dough in the pan and crimp the edges.
Cut a piece of parchment to lay on top of the tart bottom and fill with pie weights or dried beans. Bake for 10-12 minutes until it is dry and chalky white. Remove the beans and set aside to cool.
For the Filling
Heat the butter in a large skillet over medium low heat. Add the onions and cook about 30 minutes until they are very soft and caramelized. Add salt and pepper to taste.
Preheat the oven to 325 degrees F and prepare the custard.
Combine the egg with the egg yolk in a small bowl with a whisk.
Add the milk, cream, salt, pepper and nutmeg, whisking gently until combined. Do not over whisk and create bubbles or froth/foam.
Spread the onions on the bottom of the prepared tart shell and pour the custard over the onions filling the shell completely.
Bake for 30 minutes until the custard is just set. Do not let the custard rise, as this is a sign of overbaking.
Serve warm or at room temperature. Goes great with a field green salad and light vinaigrette.