Step 1

Place a rack in middle of oven; preheat to 400°. Cook 12 oz. wide egg noodles in large pot of boiling salted water, stirring occasionally, until very al dente, about 2 minutes. Drain noodles; reserve pot.

Step 2

Remove dark green parts from 1 bunch scallions and thinly slice; set aside. Thinly slice white and pale green parts.

Step 3

Melt 4 Tbsp. unsalted butter in reserved pot over medium heat. Cook white and pale green parts of scallions, 1 large onion, finely chopped, and 1 head of garlic, cloves separated, coarsely chopped, stirring occasionally, until softened but not browned, 6–8 minutes. Add ¼ cup all-purpose flour and cook, stirring constantly, until flour is pale golden, about 3 minutes. Pour in 4 cups water, whisking constantly to dissolve flour, and bring to boil. Cook, whisking often, until sauce is slightly thickened, about 5 minutes; remove from heat.

Step 4

Add zest of 1 lemon, 8 oz. cream cheese, cut into 1″ pieces, room temperature, 8 oz. Monterey Jack or pepper Jack cheese, coarsely grated, ½ cup yellow or white miso, ¼ cup chili sauce (such as sriracha), 2 Tbsp. chicken bouillon paste, and 1 Tbsp. soy sauce, preferably low-sodium, to sauce; whisk vigorously until incorporated and cheese is fully melted.

Step 5

Gently stir noodles, three 5-oz. cans oil-packed tuna, drained, and 1½ cups frozen peas into sauce; taste and season with salt if needed. Transfer mixture to a 13×9″ baking dish and level surface. Scatter 4 oz. potato chips, coarsely crushed, over in an even layer. Bake casserole until hot throughout and chips are beginning to brown, 15–20 minutes. Top with more lemon zest and reserved dark green parts of scallions; season with pepper.