These little puff pastry bites are pinch hitters when it comes to entertaining because they're as fitting at a holiday cocktail party as they are at an Easter brunch. I make the fennel filling and assemble them up to 2 days ahead of time and freeze them, then bake them (just a bit longer than listed below if frozen) as guests arrive.
green tops removed and reserved
Heat the oven to 400°F and arrange a rack in the middle.
Prepare the fennel by quartering it lengthwise, removing the core, and then slicing the fennel quarters crosswise into thin (about 1/8-inch) slices. Set aside until ready to use. (If preparing ahead of time, store the fennel in ice water until ready to use.)
Heat butter in a large frying pan over medium-high heat. When foaming stops, add the fennel (drained and patted dry if it was in ice water) , garlic, and leeks, season well with salt and freshly ground black pepper, thyme, and sugar and stir to coat.
Cook, stirring occasionally, until fennel is golden brown and fork tender. Remove from heat and set aside to cool slightly. (This can be done up to 3 days in advance.)
On a clean work surface, lay out the sheet of puff pastry. Using a 1 1/2-inch round cutter, cut the pastry into about 24 rounds, and lay flat on a parchment-lined cookie sheet.
Place one rounded teaspoon of fennel on each round, top with 1 tablespoon grated cheese, and another small rounded teaspoon of fennel.
Bake in the oven for 15 to 20 minutes, until the tops of the tartlets are golden brown, and the puff pastry is cooked through. Top each tartlet with a small dollop of Mascarpone and a pinch of fennel fronds or micro greens. Serve immediately.