Preheat oven to 400°.
Grease a 12-well mini muffin pan with cooking spray.
Line a rimmed baking sheet with paper towels.
Place potatoes on pre-pared baking sheet and let thaw.
Press all excess moisture out of potatoes.
In a medium bowl, stir together thawed potatoes, Parmesan cheese, green onion, olive oil, ½ teaspoon salt, and pepper.
Spoon 1½ tablespoons potato mixture into wells of prepared pan.
Press potato mixture into bottom and up sides of wells.
Bake for 20 minutes.
Add a Brie piece to each potato cup.
Bake until Brie melts and potato cups are crispy and golden brown, approximately 5 minutes more.
Meanwhile, in a medium skillet, cook bacon over medium heat until crisp.
Drain on paper towels.
Crumble when cool.
Reserve ½ teaspoon bacon grease in skillet.
Add apple, fennel, red onion, brown sugar, vinegar, and remaining ¼ teaspoon salt.
Sauté until soft and fragrant, 3 to 5 minutes.
Gently remove potato cups from pan.
Top each potato cup with crumbled bacon and apple mixture.
Garnish with fennel fronds, if desired.
Serve immediately.