Preheat oven to 350°F.
In large pot, cook pasta in chicken broth until al dente, according to package directions. Reserving broth, drain pasta. Pour pasta into buttered 9x13-inch baking dish. Crumble bacon over pasta.
While pasta is cooking, prepare sauce. In large saucepan or Dutch oven, melt 5 tablespoons butter over medium heat. Add onion and garlic and cook 1-2 minutes. Whisk in flour and cook 1 minute, stirring occasionally. Slowly whisk in heavy cream and stir constantly until thickened. Add cream cheese and Montreal chicken seasoning. Stir for 2 minutes until cream cheese melts down. Slowly add in 3 cups of reserved chicken broth.
Lower heat and continue stirring. Add shredded cheese 1 cup at a time. Keep stirring until sauce is smooth. Pour cheese sauce over pasta and bacon. Gently mix sauce so all pasta is covered.
Bake 20-30 minutes until the sauce is bubbly.