Snap the tough ends off the asparagus; cut into ½-inch pieces. Cut the root ends and tough green tops from the leeks; cut the 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 ½ teaspoon salt. Cook until the asparagus is just tender, about 7 minutes, then remove from the pan and set aside.
In a large pan over high heat, bring about 3 quarts water to a boil. Add the pasta and cook according to package instructions; stir in the peas at the end. Drain and return to the pan.
Meanwhile, tear the bread into chunks and put in a food processor with ½ tablespoon of butter. Process until crumbs form.
In the medium pan over medium-high heat, melt remaining 3 tablespoons of butter. Add the flour and thyme; stir until smooth and bubbling, about 30 seconds. Slowly whisk in the milk and stir to boiling and thickened, 5 to 8 minutes. Add the lemon zest, mustard, pepper, and remaining ½ teaspoon of salt. Remove from heat and add the goat cheese and 1 cup Romano; stir until smooth.
Pour the sauce over the drained pasta and peas. Add the asparagus mixture and stir well. Scrape the mixture into a 2½-quart baking dish and spread level. Sprinkle evenly with the remaining ½ cup Romano and the bread crumbs. Bake until the sauce is bubbling and the bread crumbs are browned, 15 to 20 minutes.