Mix sour cream and ice water in a bowl until combined. Set it aside.

Place flour, sugar, and kosher salt into a food processor. Pulse 3-4 times to combine.

Scatter pieces of butter on the flour mixture and process for 10 1-second pulses until incorporated with small pieces remaining.

Pour the sour cream mixture over the top and pulse 6 to 7 1-second pulses.

Pinch the dough with your fingers. If it holds together, you are done. If not, add 1 tablespoon of ice-cold water and pulse until large clumps form, about 3 to 5 pulses.

Transfer the dough onto a counter, shape into a 4-inch disk, and wrap tightly with plastic wrap. Refrigerate for at least 1 hour or overnight.

Before rolling it out, let it sit on the counter for 10 minutes.

Preheat the oven to 375°F (191°C).

Lightly flour your work surface and roll the dough into an 11-inch circle. Transfer to a 9-inch tart pan and tuck excess dough underneath to form an even edge.

Cover with plastic wrap and place in the freezer for 15-20 minutes until chilled and firm.

Place parchment paper over the dough and fill with pie weights or dry beans. Bake for 30 minutes.

Remove the pie weights and parchment paper, and bake for an additional 5 minutes until lightly browned. Set aside and do not turn the oven off.

Heat olive oil in a large skillet over medium heat. Add sliced onions and cook for 15-20 minutes until golden brown, stirring often.

Add baby spinach and cook until wilted, about 5 minutes. Set aside to cool.

In a small bowl, mix heavy cream, eggs, kosher salt, and black pepper.

Scatter crumbled goat cheese, caramelized onion, and spinach mixture over the pie crust.

Pour the heavy and egg mixture over it.

Bake for 30-35 minutes. Allow to cool for 10-15 minutes, slice, and serve while warm.