Preheat your oven to 400°F.
Melt 3 Tbs. butter in a wide skillet. Add the minced shallot and a small pinch of salt; sauté about three minutes until it starts to brown.
Add the flour and whisk until it becomes a paste. In a thin stream add the milk, whisking as you go. Once all the milk is added, add the zest and juice from the lemon, along with another pinch of salt.
Add the cooked noodles, tuna and peas to the sauce, tossing to coat. Sprinkle the grated Gruyere over the top and bake for 25 minutes, 30 tops, until the cheese is browning in places and starting to bubble.
In the mean time, melt the remaining 3 Tbs. butter in a smaller skillet. Add the panko with a pinch of salt and brown in the butter, about three minutes until crispy.
Serve the casserole garnished with anything you like, along with the crunchy panko topping.