Heat oven to 350°. Generously butter a 1-quart baking dish.
In a medium saucepan, heat milk over medium-low heat. Bring just to a boil and remove from heat. In large saucepan, melt butter over low heat. Add flour and cook, stirring constantly with a wooden spoon for 2 minutes. Do not brown. Remove from heat.
Add 2 tablespoons of the milk to flour mixture, stirring constantly until milk is incorporated. Repeat until ½ cup of the milk has been added. Then add the remaining milk, incorporating between additions, until all of the milk has been added. Return pan to low heat.
Cook, stirring frequently, especially along corners of pan, until sauce is the consistency of a thick cream. Stir in salt and remove from heat.
While béchamel is cooking, bring a large saucepan of salted water to boil. Add pasta and cook for 9 minutes. Drain (do not rinse) and set aside.
In a large skillet, heat oil over medium heat; add onion and pinch of salt; cook, stirring, for 3 minutes.
Add mixed meats and cook 1 minute more. Add pasta and stir to combine. Remove from heat and stir in béchamel and cheese.
Transfer mixture to prepared baking dish, cover with breadcrumbs and dot with butter. Bake until golden and bubbly, about 20 minutes.
Let stand 5 to 10 minutes before serving.