Preheat oven to 400°. Snap tough ends off asparagus; cut spears into 1/2-inch pieces. Cut root ends and tough green tops from leeks; cut leeks in half lengthwise and rinse well under running water, then thinly slice crosswise.
In a medium pan over medium-high heat, stir the asparagus with the leeks, 1 tablespoon butter, and 1/2 teaspoon salt. Cook until the asparagus is just tender, about 7 minutes, then remove from pan and set aside.
In a large pan over high heat, bring about 3 quarts water to a boil. Add pasta and cook according to package instructions; stir in peas at the end. Drain and return to pan.
Meanwhile, tear bread into chunks and put in a food processor with 1/2 tablespoon butter. Whirl until crumbs form.
In the medium pan over medium-high heat, melt remaining 3 tablespoons butter. Add the flour and thyme; stir until smooth and bubbling, about 30 seconds. Slowly whisk in milk and stir until boiling and thickened, 5 to 8 minutes. Add lemon peel, mustard, pepper, and remaining teaspoon salt. Remove from heat and add goat cheese and 1 cup romano; stir until smooth.
Pour sauce over drained pasta and peas. Add asparagus mixture and stir well. Scrape mixture into a 2 1/2-quart baking dish and spread level. Sprinkle evenly with remaining 1/2 cup romano and the bread crumbs. Bake until sauce is bubbling and bread crumbs are browned, 15 to 20 minutes.
Note: Nutritional analysis is per serving.