Preheat the oven to 400°.
In boiling water, cook 2 cups (about 8 ounces) of pasta to al dente according to package instructions and drain well.
Mix the topping of ½ cup panko breadcrumbs, ⅓ cup grated Parmesan cheese, and 2 tablespoons melted butter.
Chop 1 rib of celery and ½ medium onion. Sauté in 1 teaspoon of butter over medium-high heat until they become slightly transparent—about 5 minutes.
In a large bowl, mix drained cooked noodles, the sautéed vegetables, 1 can cream of celery (or chicken or mushroom) soup, ½ cup milk, 1 cup frozen peas, salt, and pepper to taste, and two (5-oz) cans of tuna, drained. Gently stir to avoid damaging the pasta.
Pour the mixture into a 6x9 or 8-inch round baking dish. Sprinkle the topping over the casserole.
Bake until nicely browned—about 30 minutes. Remove and allow to sit for 5 minutes before serving.
