Preheat your oven to 400 F and line baking sheets with parchment. Thinly slice the zucchini and summer square. A handheld mandolin makes this super easy in 1 to 2 minutes—half the cherry tomatoes.
Herbed Sauce
Mix creme fraiche with fresh herbs and minced garlic. Taste and season with salt and pepper as desired. Set aside.
Savory Tartlets
Unfold the puff pastry and roll slightly to even the sides. With a pastry cutter or pizza wheel, cut each sheet into squares, using the fold lines as guides.
Score each square with a ½-inch border inside the pastry edge with a sharp knife. Be careful not to cut all the way through. With a fork, prick the inside square of each pastry.
Spread a dollop of herbed creme fraiche over the inner square of each pastry. Place your choice of thinly sliced vegetables in the center. Drizzle with olive oil and season with salt and pepper.
Brush the edges of each square with the beaten egg and bake for 12-14 minutes, or until the pastry has puffed up around the edges and is golden brown.
Notes
Make Ahead: To make these tartlets in advance, assemble the appetizers and refrigerate several hours before serving. When ready to serve, bake for 12 to 14 minutes. They can also be served at room temperature, but it's important to not reheat them before serving.
Serving: To serve, I like to pipe small dollops of the remaining herbed creme fraiche on the top of each tart using a pastry bag and a star tip. Fresh basil leaves finish them beautifully.