01 - Preheat oven to 400°F. Line a rimmed baking sheet with parchment paper.
02 - Roll thawed puff pastry onto prepared baking sheet. Fold edges inward by 1 inch to form a raised border. Prick center base all over with fork tines.
03 - Bake pastry for 10 minutes until lightly golden. Remove from oven and carefully press down any puffed center areas to create an even base.
04 - Whisk together eggs, milk, heavy cream, Dijon mustard, salt, black pepper, and smoked paprika in a large bowl until completely smooth and frothy.
05 - Distribute chopped bacon (or mushrooms), spinach, cherry tomatoes, and green onions evenly over par-baked pastry base. Sprinkle cheddar cheese across the top.
06 - Pour egg mixture slowly and evenly over layered fillings. Return to oven and bake for 25 minutes until center is just set and top is golden brown.
07 - Let tart cool on baking sheet for 5 to 10 minutes to allow custard to set. Scatter fresh chives over top, slice into wedges, and serve warm.