Preheat oven to 400˚F (200˚C). Melt 2 tbsp (30 mL) butter in large skillet set over medium heat. Cook ham and onions 3 to 4 min. until onions have softened. Sprinkle in flour. Cook, stirring, 2 to 3 min. until flour and butter are smooth.
Slowly whisk in milk. Continue to whisk while bringing to a boil. Reduce heat to medium-low. Cook, stirring constantly, 7 to 8 min. until sauce is slightly thickened. Remove from heat.
Stir in 1 cup (250 mL) St. Paulin cheese, smoked Oka, salt and pepper. Mix in pasta; spread in greased 8-cup (2-L) baking dish. Sprinkle with remaining St. Paulin cheese. Melt remaining butter; toss with breadcrumbs. Sprinkle overtop.
Bake 20 min. or until golden and bubbly. Let stand 10 min. before serving.
For vegetarian mac & cheese, switch the ham for chopped broccoli or cauliflower.