Preheat your oven to 375°F.
Prepare a baking dish by lightly greasing it with butter.
In a saucepan, combine the stock, cream, wine, tomato puree, and saffron.
Bring it to a simmer to let the flavors meld together.
Melt butter in a large skillet over medium heat.
Add the leeks and garlic, sautéing until softened.
Quickly toss in the shrimp and scallops, cooking for just a minute.
Place the seafood mixture evenly in the greased baking dish.
Gently pour the creamy sauce over the seafood and add the crabmeat.
Mix the Gruyere cheese and breadcrumbs together, then sprinkle evenly on top.
Bake for 25-30 minutes or until the topping is golden and bubbling.