Mexican Filling
Halve tomatoes lengthwise. Place on parchment paperlined baking sheet; sprinkle with oregano, oil, salt and pepper. Toss lightly; spread in single layer. Roast in centre of 400°F (200°C) oven until wrinkly, about 15 to 20 minutes. Let cool; add corn, peppers and green onions. Toss and set aside.

Oka Cheese Sauce and Macaroni
In large saucepan, melt butter over medium heat. Fry onion and garlic, stirring until translucent, about 6 minutes. Stir in flour; cook, stirring for 4 minutes. Whisking constantly, pour in milk; whisk until sauce is creamy and thickened. Whisk in mustard, salt, pepper, then Oka and Parmesan, whisking until cheese has melted. Meanwhile, cook macaroni in salted boiling water until tender, about 8 minutes. Drain; add to cheese sauce and mix. Scoop 2 cups of the macaroni mixture into 3 greased, 8-inch (20 cm) square baking dishes. Spread evenly. Over each dish, sprinkle a third of the Mexican filling. Spread a third of remaining macaroni mixture over each tomato layer. (Make-ahead: wrap in heavy-duty foil and freeze for up to 1 month. Let thaw in refrigerator and add 10 minutes to baking time.)

Topping
Combine corn chips and shredded Oka. Sprinkle evenly over all 3 dishes. Cover with foil; bake in centre of oven at 350°F (180°C) for 30 minutes. Uncover and bake until browned and bubbling, about 10 minutes. Do not apply topping to portions to be frozen. Wait until final stages of baking.