A crusty, cheesy topping, giant shells and half pulses make a healthier take on a homey classic.
In a small saucepan, melt the butter until foaming. Whisk in the flour until smooth.
Add the milk in a steady stream while whisking to remove any lumps. Add the thyme sprigs and cook over medium heat until the sauce has simmered for 3 minutes.
Remove from heat and stir in ½ the cheese (reserving the other half for the baking step)
Set aside.
In a sauté pan, heat the butter until foaming. Add the mushrooms and onions and cook over high heat until nicely brown.
Add the garlic and the celery and sauté a few more minutes until aromatic.
Add the white wine and reduce just until there is no visible liquid in the bottom of the pan. Reserve.
In a large pot of boiling salted water cook the giant shells7-8 minutes or until al dente (still some resistance to the tooth). Pour through a colander and run some cold water over the shells to cool them. Allow to drain thoroughly.
In a medium bowl, combine the drained tuna, white beans, cheese sauce, mushroom mix and the herbs. Taste for seasoning and adjust if necessary.
Preheat the oven to 375°F
Butter a 9x13 casserole dish and smear the bottom with ¾ of the tomato sauce.
Fill each of the shells completely with the tuna filling and place on the tomato sauce.
Drizzle the casserole with ½ cup of water. Dot with remaining tomato sauce and sprinkle with cheese.
Mix the Panko olive oil and a pinch of salt and pepper. Sprinkle over the cheese.
Cover with aluminum foil and bake for 30 minutes. Remove the tin foil and continue to bake for another 10-15 minutes until the cheese and Panko are brown and crusty.