Preheat your oven to 180ºC (350ºF).

Cook the pasta shells according to the package instructions. Once cooked, drain them and set aside to cool.

Wilt the spinach in a pan with about 100ml of water. Let the water boil, then remove from heat. Add the spinach and let wilt for a minute. Use a slotted spoon or tongs to remove the spinach from the water. Let the spinach cool and squeeze out the excess water with a paper towel.

In a small bowl, mix the mascarpone, red pepper sauce, and wilted spinach. Once the shells have cooled, stuff them with the mascarpone filling.

Sweat the onion and garlic in a tablespoon of olive oil until soft and translucent. Add the dried herbs and mix well. Pour in the passata and vegetable stock, and let simmer for about 10 minutes until the sauce thickens slightly.

Spoon the sauce into the bottom of a skillet or oven-safe baking dish. Place the stuffed shells in the dish. Sprinkle the shredded mozzarella on top and optionally, some thyme leaves.

Bake in the preheated oven for about 15 minutes, until the cheese is melted and just starting to turn golden. Use a spoon to divide the stuffed shells between plates.