Preheat oven to 350°F. Cook pasta al dente, according to package directions. Drain and set aside.
Cook potatoes in pot of boiling water until fork tender. Allow to cool completely. Peel with pairing knife and shred with box grater. Set aside.
Heat large pot over medium-high heat. Add bacon and cook until crispy and browned. Remove with slotted spoon and transfer to plate lined with paper towels. Remove all but 1 tablespoon of bacon grease from pot. Stir in onions and garlic and sauté for about 5 minutes, until soft and translucent. Add cabbage and leeks. Season with 3/4 teaspoon salt, 3/4 teaspoon black pepper and red pepper flakes. Stir and continue cooking about 5 minutes more until mixture is soft. Fold in shredded potatoes and remove from heat. Transfer to large bowl and set aside.
Place milk in medium saucepot and bring to barely a simmer, over medium heat.
Return large pot to stove and place over medium-high heat. Add butter and melt. Whisk in flour and cook about 10 seconds. Carefully add warm milk, while whisking continuously until all of milk has been added and butter/flour mixture has completely dissolved. Lower heat to medium and cook until thickened. Season with 3/4 teaspoon salt, 3/4 teaspoon black pepper and paprika. Once thick, remove from heat.
In medium bowl, toss together monterey jack, cheddar and havarti. Add cheeses to hot thickened milk, reserving about 1/2 cup for topping. Stir with wooden spoon, until cheese has melted. Fold in pasta, cabbage mixture, bacon and scallions. Pour mixture into 9x13-inch baking dish or into individual 8-ounce ramekins. Top with breadcrumbs and reserved cheese. Bake 25 to 30 minutes, until bubbly. Place under broiler a few seconds to brown and crisp the top. Serve warm and garnish with sliced scallions on top.