01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Roll out puff pastry onto prepared baking sheet. Fold edges inward to create a 1-inch border. Prick base thoroughly with fork.
03 - Bake pastry for 10 minutes until lightly golden. Remove from oven and gently press down any puffed areas to create flat base.
04 - Whisk together eggs, milk, heavy cream, Dijon mustard, salt, black pepper, and smoked paprika in large bowl until completely smooth and incorporated.
05 - Distribute cooked bacon (or mushrooms), chopped spinach, halved cherry tomatoes, and sliced spring onions evenly over pastry base. Sprinkle shredded cheddar cheese across top.
06 - Pour egg mixture gently over fillings, spreading evenly. Return to oven and bake for 25 minutes until center is just set and top is golden brown.
07 - Cool tart for 5-10 minutes to allow filling to set. Garnish with chopped fresh chives, slice into wedges, and serve warm.