Peel and wash the spinach thoroughly in several changes of water. Drain well, then cook in a large covered saucepan over medium heat without added water. Season with salt and pepper at the end, drain, and set aside.
Peel the carrots and cut into sticks. Blanch in boiling salted water for 5 minutes, then cool under cold water and set aside.
In a sauté pan, reduce the raspberry vinegar to about 1 teaspoon of liquid. Add the poultry fumet and reduce by half. Stir in the cream, season with salt and freshly ground pepper, and simmer for 2 minutes.
Brush the puff pastry rectangles with beaten egg. Bake in a preheated oven at 200°C (400°F) for 15 minutes until golden.
Open the baked puff pastries and fill with the spinach and carrots. Drizzle with the sauce and garnish with chopped chives.
For variety, substitute spinach and carrots with cauliflower and broccoli—equally delicious.
