Trim the rind from the brie and cut it into smaller pieces. Grate the Swiss cheese.
In a large pot of salted water, cook pasta according to package directions for al dente. Drain and set aside.
Melt butter in a large pot over medium-low heat. Add garlic and cook until sizzling. Stir in the flour, salt, pepper and dry mustard. Whisking, cook until the mixture starts to bubble.
Pour in the milk. Stirring, bring to a boil over medium-high heat.
Add the brie, Parmesan and Swiss cheeses. Whisk until smooth.
Add the cooked pasta to the cheese sauce. Transfer to a 9x13 baking dish or 6 individual crocks.
In a small bowl, combine the crushed Ritz crackers and melted butter. Sprinkle evenly over top of the mac and cheese. Bake 15-20 minutes, until top is golden brown and bubbling.