Preheat your oven to 400F. Lightly butter a 9x13 baking dish.
In a heavy bottom saucepan, heat milk over medium heat. Stir in pasta. Cook until just tender. Drain the pasta reserving 2 cups of the milk for the sauce.
In a large skillet, heat ½ tablespoon oil on high heat. Add chorizo and sauté 2 to 3 minutes before adding poblanos. Sauté until chorizo is browned and peppers are slightly tender. About 7-8 minutes.
In a dutch oven or large, deep sided skillet make the roux. Melt the butter over medium heat, add flour to the melted butter, stirring for 3 to 4 minutes.
Slowly add 2 cups of the reserved pasta milk, stirring to incorporate, scraping the bottom of the pan. The roux will clump but continue to stir and the clumps will dissolve. Switch to a whisk to remove any remaining clumps.
Add the fresh grated cheese, one handful at a time, whisking until the cheese has melted before adding more. Once the cheese is incorporated, add the salt and ground mustard.
Stir in the poblanos and chorizo followed by the pasta. Stir well to evenly coat all pasta with cheese sauce and distribute the chorizo and peppers.
Transfer to the 9x13 baking dish. Top with breadcrumb topping and baking until cheese is bubbly and breadcrumbs are browned, about 10 to 15 minutes.