To make the harissa, preheat the oven to 200°C (390°F). Place the red bell pepper on a baking tray lined with parchment paper and roast, turning once, until the skin is blackened, about 20 to 25 minutes. Transfer to a bowl, cover tightly (or seal in a bag) and let cool. Peel off the skin and discard the stalk and seeds.

While the pepper roasts, toast the cumin and coriander seeds in a dry frying pan over medium heat, stirring, until fragrant, 1 to 2 minutes. Let cool, then grind to a powder in a food processor. Add the roasted pepper, chopped chilies, garlic, salt and olive oil, and process to a smooth paste. Set aside.

To make the tarts, toss the pumpkin slices with the olive oil and a little salt and pepper. Spread in a single layer on a second lined baking tray and roast alongside the pepper for 20 to 25 minutes, until tender. Let cool slightly.

Reduce the oven to 190°C (375°F).

Cut the feta in half. Cut one half into small cubes and crumble the other half.

In a small bowl, mix 1 tablespoon of the harissa with the yogurt, crumbled feta and beaten egg until combined.

Cut each puff pastry sheet into 4 squares (8 total) and arrange on lined baking trays. Spread each square with the harissa mixture, leaving a ½-inch (1 cm) border. Top with the roasted pumpkin slices and feta cubes, then sprinkle with the thyme leaves.

Bake for 15 to 20 minutes, until the pastry is puffed and golden and the bottoms are crisp. Serve warm or at room temperature.