Step 1
Place the potatoes and carrots into a large pot and cover with salted water. Bring to a boil over high heat then reduce heat to medium-low cover and simmer until tender about 15 minutes. Drain vegetables then cover to keep warm.
Step 2
Preheat an oven to 350 degrees F (175 degrees C). Spray a baking dish with cooking spray.
Step 3
Melt 1/4 cup butter in a large skillet over medium heat. Stir in onion and garlic; cook and stir until the onion is softened about 3 minutes.
Step 4
Sprinkle in the flour and and reduce heat to medium-low. Continue to cook and stir for about 1 minute whisking out any lumps.
Step 5
Whisk in the milk until the mixture thickens 5 to 7 minutes.
Step 6
Flake the tuna then stir it into the white sauce along with the potatoes carrots and peas. Season with salt and black pepper.
Step 7
Transfer the tuna mixture to the prepared baking dish.
Step 8
Mix 1/4 cup of melted butter with the crackers together in a bowl.
Step 9
Sprinkle the cracker mixture over the tuna and potatoes.
Step 9
Bake in the preheated oven until the sauce is bubbly about 30 minutes.