When it comes to radiating warmth at the dinner table, nothing beats a one-pot casserole. What's more, this dish can be assembled a day ahead, goes straight from the oven to the table, and offers help-yourself allure in abundance.
Preheat oven to 375 degrees. Cook pasta in a large pot of salted boiling water 4 minutes less than instructed on pasta package. Reserve 1/2 cup pasta water. Add peas and drain immediately.
Melt butter in a medium saucepan over medium-high. Add flour; cook, stirring, 1 minute. Whisk in milk. Gently simmer, stirring constantly, until slightly thickened, about 7 minutes. Combine pasta mixture, milk sauce, salmon, lemon zest, scallions, dill, and reserved pasta water in a bowl. Season with salt and pepper.
Transfer to a 2 1/2-quart baking dish. Cover with parchment-lined foil; bake until salmon is just opaque, 20 to 25 minutes. Serve, with lemon wedges, more scallions and dill, and breadcrumbs.