Bring a large pot of salted water to a boil over high heat; cook pasta until al dente (10-12 minutes); drain and set aside.
In a small saucepan over low heat, bring the milk just to a simmer (or scald).
In large saucepan over medium heat, melt 4 tablespoons butter; add in onion and cook 3-4 minutes until soft and fragrant but not colored.
Decrease heat to med-low; add in flour; cook, stirring, about 3 minutes.
Add in scalded milk; increase heat to medium; bring mixture to a boil.
Simmer, whisking, for 3-5 minutes, until thickened and smooth with chunks of onion.
Remove pan from heat and gradually stir in the ribiola, fontina, and Taleggio, in batches, until the cheeses are incorporated (they need not melt entirely); add in pasta and stir to combine; season with pepper.
Butter a 4 quart shallow baking dish; add the pasta; sprinkle with gorgonzola and bread crumbs; dot with the remaining 2 tablespoons butter.
Bake on the middle oven rack 25 minutes, until bubbling.
Remove from oven; increase oven temperature to broil; place dish about 4 minches from flame; broil 1-2 minutes until golden.