Prep time: 45 minutes
1. To make the dough: preheat the oven to 400F/200C. In a food processor, grind the hazelnuts to coarse bits. Add both flours and the salt and process to mix, then whirl in the oil. Add 7 tbsp. (105 ml) of the ice water and pulse just until the dough is lumpy looking. Scrape the dough out onto the floured counter, press it together. Don’t overwork the dough. On the floured counter, roll each disk out to a 7-in (17-cm) round and transfer them carefully to a large, rimless baking sheet/tray covered with parchment paper or silicone baking sheet. Refrigerate.
2. To make the filling: Heat a large frying pan over high heat. Add the oil and onions and stir, reducing the heat to medium as they start to sizzle. Add the eggplant/aubergine, squash, and zucchini/courgette. Sauté, stirring constantly, to soften vegetables, about 5 minutes. Add the scallions/spring onions and rosemary. Cook, reducing heat to medium-low if the vegetables stick to the pan, until the vegetables are browned and shrunken. Do for about 5 minutes, until the mixture is thick. When done, take the pan off the heat and let the filling cool to room temperature.
3. Remove the dough rounds from the refrigerator and mound one fourth of the vegetables in the center of each round, leaving a 1½-in (4-cm) border. Fold the dough over the filling and loosely pleat it, being careful not to tear it. Bake the tarts until the crusts are quite browned and crisp, about 20 minutes. Slide the tarts onto a serving platter or cutting board and let them stand for 5 minutes to firm up. Top the vegetables in the center with the basil and cut each tart into four wedges to serve one tart per plate.