Preparation

1. ​Preheat and Blind Bake: Preheat your oven to 375°F (190°C). Fit one pie crust into your 9-inch pie plate, crimp the edges, and prick the bottom with a fork. Line with parchment paper, fill with pie weights or dried beans, and bake for 15 minutes. Remove paper and weights, and bake for 5 more minutes until dry. Let cool slightly, then brush the bottom lightly with Dijon mustard.
2. ​Caramelize the Onions: While the crust bakes, melt the butter and olive oil in a large skillet or Dutch oven over medium-low heat. Add the sliced Vidalia onions and toss to coat. Cook low and slow, stirring occasionally, for 25–30 minutes until the onions are soft, translucent, and deeply golden. In the last 5 minutes, stir in the fresh thyme, brown sugar, and balsamic vinegar to build rich, savory depth. Remove from heat and let cool.
3. ​Mix the Custard: In a large bowl, whisk together the 5 large eggs, whole milk, heavy cream, kosher salt, and black pepper until smooth.
4. ​Assemble the Pie: In a bowl, toss together the sharp white cheddar and Gruyère cheeses. Scatter half of the cheese blend across the bottom of the blind-baked crust. Spread the caramelized Vidalia onions evenly over the cheese. Top with the remaining cheese blend. Pour the egg and cream custard mixture evenly over the filling.
5. ​Bake: Place the pie on a baking sheet to catch any drips. Bake at 375°F for 35–45 minutes, or until the custard is completely set in the center and the top crust/cheese is bubbly and golden brown. (If the edges begin to brown too quickly, cover them loosely with a ring of aluminum foil).
6. ​Rest and Serve: Remove from the oven and let the pie cool for at least 20 to 30 minutes before slicing. This allows the savory custard to set firmly. Garnish with a few fresh thyme leaves and serve warm or at room temperature alongside a crisp green salad.