In a large skillet, melt 2 tablespoons of butter over medium heat. Add thinly sliced yellow and red onions, stirring occasionally. Cook for about 25-30 minutes until the onions are deep golden brown and caramelized. Once done, remove from heat and set them aside to cool.

While the onions cool, preheat your oven to 375°F (190°C).

Take the pie crust out of the fridge and gently roll it out to fit your tart pan. Blind bake the crust for 10 minutes until lightly golden.

Spread the caramelized onions evenly across the pre-baked crust. Sprinkle Gruyère cheese over the top.

Beat an egg and brush it over the crust’s edges. This gives the tart a beautiful golden finish.

Bake the tart for about 30 minutes until the cheese is bubbling and the crust turns golden brown.

Let the tart cool for about 10 minutes before serving warm or at room temperature.