Heat oven to 375°F. Lightly grease a 9x13-inch baking dish.
Boil noodles in salted water until just shy of al dente, 1–2 minutes less than package time. Drain.
Melt 2 tbsp butter in a skillet over medium heat. Soften onion and celery, 4–5 minutes. Remove from heat.
Whisk soup, milk, sour cream, lemon juice, salt, pepper, and garlic powder in a large bowl. Stir in paprika if using.
Fold in noodles, tuna, peas, sautéed vegetables, and half the cheddar. Toss gently to coat.
Spread mixture into the prepared dish and sprinkle with remaining cheddar.
Combine breadcrumbs, remaining 2 tbsp melted butter, and parsley. Scatter evenly over the top.
Bake until bubbling at the edges and the topping is golden, 20–25 minutes.
Rest 5 minutes before serving. Garnish with extra parsley if you like.