Preheat your oven to 425°F. Lightly grease a 9x13 inch baking dish.

Melt butter in a large saucepan over medium heat until foaming. Add the sliced leek and cook, stirring occasionally, for 3 minutes until softened.

Stir in the all-purpose flour and cook, stirring with a wooden spoon, for 1 minute until the mixture bubbles and forms a roux.

Gradually add the milk, about 1/2 cup at a time, whisking constantly to prevent lumps. Continue to stir until all the milk is incorporated and the sauce boils and thickens, about 3 minutes.

Stir in the Dijon mustard until well combined. Add the drained and flaked tuna, frozen peas, frozen corn, and baby spinach. Cook, stirring, for 2 minutes or until warmed through and the spinach wilts.

Remove from heat. Stir in the green onions and 1 cup of the shredded cheese blend. Continue to stir for 1 minute until the cheese melts into the sauce.

Transfer the tuna mixture to the prepared baking dish and spread evenly.

Arrange the thinly sliced potatoes over the tuna mixture, slightly overlapping them to create an even layer.

Sprinkle the remaining 1/2 cup of shredded cheese over the potatoes.

Bake for 45 minutes or until the potatoes are golden and tender, and the cheese is bubbly and lightly browned. If desired, broil for the last 1-2 minutes for extra crispness.

Remove from the oven and let stand for 10 minutes before serving to allow the casserole to set.