1/2 ounce dried wild mushrooms
2 cups whole milk
2 tablespoon butter
4 tablespoons flour
2 1/2 cups chicken or vegetable stock
4 5-ounce cans water-packed tuna, drained
12 to 16 ounces pasta — Manischewitz brand wide yolk-free noodles or a sturdy pasta shape like rigatoni
2 cups frozen peas
8 ounces (about 2 cups) shredded part-skim mozzarella, divided
2 teaspoons Italian seasoning
1 to 2 tablespoons Worcestershire sauce
2 1/2 teaspoons kosher salt
Salt and pepper to taste
Cover the mushrooms with 1/2 cup of boiling water and set aside to steep for at least 15 minutes. Drain the mushrooms, reserving the liquid. Roughly chop the mushrooms.
Warm the milk for 1 minute in the microwave and set aside. In a small pan over medium heat, melt butter, then add the flour. Whisk to combine (it will clump up and look crumbly) and then cook for about 30 seconds. Very slowly pour the milk into the butter-flour mixture, whisking to combine — it will at first look lumpy, then like mashed potatoes, then it will thin out to a milk-like consistency. Simmer for several minutes, stirring frequently, until the mixture has thickened to the consistency of buttermilk and coats the back of a spoon. Do not let the mixture boil. Remove the pan from heat and stir in the chicken stock and mushroom-soaking liquid.
Empty the tuna into a large bowl and use a fork to break up any big chunks. Pour the sauce over top, then add the dry pasta, chopped mushrooms, frozen peas, 1 cup of the cheese, Italian seasoning, 1 tablespoon of Worcestershire sauce, and salt. Stir to combine. Pour everything into a greased 6-quart (or larger) slow cooker, and cook on LOW for 2 to 3 hours.
About 15 minutes before the end of cooking, sprinkle the remaining 1 cup of cheese over the top of the casserole. (Do not stir the casserole.) Cover and cook until the cheese on top has melted. (If the casserole seems a bit liquidy as you're adding the cheese, leave the lid propped during this last 15 minutes to allow some evaporation.)