Peel and cube the potatoes and carrots.

Place the potatoes and carrots into a large pot and cover with salted water.

Bring to a boil over high heat.

Reduce heat to medium-low, cover, and simmer until tender, about 15 minutes.

Drain vegetables and cover to keep warm.

Preheat an oven to 350 degrees F (175 degrees C).

Spray a baking dish with cooking spray.

Melt 1/4 cup butter in a large skillet over medium heat.

Stir in chopped onion and crushed garlic.

Cook and stir until the onion is softened, about 3 minutes.

Sprinkle in the flour and reduce heat to medium-low.

Continue to cook and stir for about 1 minute, whisking out any lumps.

Whisk in the milk until the mixture thickens, 5 to 7 minutes.

Flake the drained tuna.

Stir the tuna into the white sauce along with the potatoes, carrots, and frozen peas.

Season with salt and black pepper to taste.

Transfer the tuna mixture to the prepared baking dish.

Melt 1/4 cup of butter.

Mix melted butter with the crushed buttery round crackers in a bowl.

Sprinkle the cracker mixture over the tuna and potatoes.

Bake in the preheated oven until the sauce is bubbly, about 30 minutes.