Butter 1 quart square baking dishes with two tbsp of butter. Cook the pasta for 2 min less than the package directions suggest, about 6-8 min; drain and return it to the pot.
In a small saute pan, melt 2 tbs of butter. Add cracker and bread crumbs. Stir until crumbs have fully absorbed the butter. Remove from heat and let cool. In a bowl, mix together parsley, french fried onions and Parmesan cheese. Add cooled crumb mixture. Set aside.
Crisp up bacon in a large pot. Remove bacon pieces with a spoon and set aside. Pour off all drippings.
Heat a tbs of olive oil to a medium sized sauce pot. Saute onions and mushrooms until unions are tender, about 3-mins. Add garlic stirring often until softened. Remove from pan and return pan to heat. Melt remaining butter in same pot over medium heat. Sprinkle flour over butter and cook 2 to 3 minutes on medium heat, whisking until a roux or paste forms. Add cold milk and whisk vigorously until dissolved. Cook sauce on medium-low heat until thick and bubbly. Add heavy cream, all cheeses, salt, pepper, and nutmeg. Cook until cheeses are fully melted, stirring occasionally.
To the sauce, add the mushroom mixture, pasta and crispy bacon; stir to combine well. Mixture will be slightly wet. Divide evenly into prepared baking dishes and sprinkle with cracker topping.
You can freeze this to prepare later on. Just cover completed unbaked mac n' cheese with plastic wrap, then foil, and freeze for up to 3 months. To bake, let thaw overnight in frig. Remove plastic wrap then re-cover with foil, and bake at 350F until heated through, about 50-55 minutes. Uncover and continue to bake until top is golden, 5-10 minutes.
To bake immediately: Bake at 350F until golden brown.