Press crust into quiche pan and line with parchment paper. Add pie weights or raw rice. Bake 20 minutes, then remove pie weights.
Bake another 5 minutes, until edges are golden brown and bottom of crust no longer looks raw.
Sauté shallots in butter for 3 minutes until lightly browned.
Whisk together eggs, cream, mustard, salt, and pepper in a large bowl.
Spread shallot slices on crust, followed by thawed spinach, and half of the gruyère cheese. Add egg custard and rest of gruyère.
Bake for 30-35 minutes at 375 degrees Fahrenheit, until lightly browned on top.