Preheat oven to 375°F.
Make 6 boxes from the puff pastry (about 4 1/2-inches square). See note below.
Bake for 20-25 minutes until deep golden-brown.
While the boxes are baking, make the ragoût.
Melt the butter in a large skillet.
Add the fennel, shallot, and 3 tablespoons water; cover with a piece of rumpled parchment and cook over low heat, stirring once or twice, until the fennel and onion soften, about 10 minutes.
Add another tablespoon of water, if needed, to keep it moist, but don't be concerned if it browns a little. That will add to the flavor and color to the ragoût.
Add the stock, cover again with the parchment, and simmer for 10 minutes.
Add the wine and saffron, cover once again with the parchment, and let simmer 5 minutes more.
Season with salt and pepper.
Add the tomatoes, peas, and langoustine tails and raise the heat to medium-high.
Cook for 3-5 minutes, stirring, until peas are cooked but still brightly colored, and the langoustine tails are warmed through.
Adjust seasoning and add cream. Bring to a simmer.
Using a small spatula, take off the tops off the inner part of the box (where you scored the pastry).
Fill the boxes with ragoût, letting some spill over onto the plate. Replace the box top and serve immediately.