Preheat oven to 375F. In a sauté pan over medium heat add 1 teaspoon olive oil, red pepper flakes and garlic. After 1 minute add fennel fronds, kale and spinach. Cook until garlic is golden and soft and greens are wilted and crisp on the ends.
Transfer contents of pan to food processor. Add 1/4 cup of olive oil and blend until a thick pesto forms. Season with salt and pepper. Set aside.
In the same pan, heat remaining 2 teaspoons oil over medium heat, add sliced fennel and onion. Cook until the mixture has wilted and caramelized. Season wit salt and pepper, transfer to a bowl and set aside.
In same pan, over medium-high heat, add bacon and cook until crisp turning once. Drain on paper towels, crumble and set aside.
On a floured board, roll out puff pastry to 1/4-inch thick. Using a 3 3/4-inch tart pan, cut out four rounds of dough. Place a dough round inside each of the 4 3 3/4-inch pans and using fingertips push down and out on dough so it reaches the edges of the pan.
Add 1 tablespoon of pesto to each tart shell. Fill with caramelized onion and fennel mixture, place on a baking sheet and transfer to oven. Bake tarts for 30 minutes, or until puffed and golden on the edges.
Top tarts with crumbled bacon.