Cut one sheet of defrosted puff pastry into 6 equal rectangles. Place each slice into a lightly greased muffin tin or individual quiche pans.
Saute bacon in a nonstick skillet over medium-high heat until crisp. Once the bacon is crisp, remove it from the pan and place it on a paper towel-lined plate. Discard all but 1 tablespoon of bacon grease, and then return the pan to the heat.
Add the potatoes, onions, and peppers to the pan. Season with ½ teaspoon kosher salt. Saute over medium heat until the potatoes are tender about 8-9 minutes. Once the potatoes are tender, add in spinach and let wilt for 1-2 minutes. Taste and adjust seasoning if needed.
Remove the sauteed vegetables from the heat and divide this mixture evenly between the six puff pastry cups. Then evenly divide the crumbled bacon and shredded cheese between the quiches.
In a medium mixing bowl or glass measuring cup, whisk three eggs with milk, ½ teaspoon salt, pepper, and cayenne. Pour this egg custard into each mini breakfast tart, dividing equally.
Fold puff pastry over the egg and potato cups to cover the filling. In a small bowl, whisk together the remaining egg with 1 tablespoon of water and then brush over each breakfast pastry.
Bake for 20-25 minutes or until browned. Let cool for 10 minutes before removing from the mini quiches from the pan and serving.
Notes
Puff Pastry: Be sure to defrost your puff pastry in the fridge overnight, not at room temperature or in the microwave, as it will lose the ability to puff.Vegetarian-Option: Omit the bacon and instead saute the vegetables in 1 tablespoon olive oil.Cheese: I love Gruyere cheese for its sharp taste and melting quality. But feel free to use Swiss or cheddar cheese. Even Parmesan cheese is a great substitute.Make-Ahead/Storage Instructions:
Up to 3 Days in Advance: Assemble and bake as directed. Let cool and then store in the refrigerator in an airtight container for up to 3 days. Reheat the pastries on a sheet pan in a 300-degree F oven for 15-20 minutes before serving.
Up to 1 Month in Advance: Assemble and bake as directed and then let the tarts cool completely. Once cooled, place them in a freezer-safe bag or freezer-safe airtight container and freeze for up to 1 month. To reheat, place frozen tarts on a baking sheet and bake at 350 degrees F for 35-40 minutes, or until warmed through.