Thinly slice the fennel and onion. Seat aside the fronds.
Heat the olive oil in a frying pan. Add the fennel and onion and cook, under cover, for 5 minutes. Add the oregano and lemon juice, season to taste, then cover again and cook for about 8 minutes until they are soft and start caramelising
Preheat the oven to 180°C. Line a baking sheet with parchment paper.
Unroll the pastry onto a flat surface. Using a pizza cutter, cut the pastry into small rectangles (or squares if preferred) – about 12 x8 cm wide.
Using a knife, create a border on each tartlet, about 1 cm from the edge.
Spread the pesto all over the tarts, up to the border, then top with the fennel and onion mixture, again spreading it out evenly to the borderline.
Put the tartlets on the lined baking sheet, then bake for 10 to 12 minutes until the pastry is cooked through and the crust is golden.
Garnish the tarts with the frond leaves, then serve them immediately with a crunchy salad, or let them cool and serve as nibbles at room temperature.