Heat olive oil and 1 tablespoon butter in a medium skillet over medium-high heat. Add onion and cook until caramel in color, about 10-15 minutes. Add pear and cranberries. Season with salt and pepper and sauté for an additional 5 minutes. Remove from pan and set aside.
Cook pasta shells in boiling, salted water 2 minutes less than package directions. Drain and set aside, reserving 1 cup of the starchy pasta water.
Meanwhile, in dry skillet, over medium-high heat, toast walnuts, until just golden brown, about 3 minutes. Set aside and preheat oven to 350°F.
In large skillet, melt remaining 4 tablespoons butter over medium heat. Sprinkle flour into melted butter, whisk to combine and cook 1 minute. Slowly pour in milk, whisking constantly, until no lumps remain. Simmer 5 minutes, stirring occasionally, until mixture thickens. Whisk in balsamic vinegar and season with black pepper. Add 3/4 of brie and blue cheeses, stirring until melted completely.
In large, rectangular casserole dish, combine onion and fruit mixture with noodles and sauce. Toss until well coated. If your sauce needs a little help sticking to the pasta, add some of that reserved pasta water to mixture. Top with toasted walnuts and breadcrumbs. Add remaining cheese to top and transfer casserole dish to oven. Bake for 30 minutes, until brown and bubbly. Serve immediately.