Pre-heat the oven to 350°F.
Place the chopped onions in a medium bowl.
Add the beef rub and mix with the onions to thoroughly combine.
Add the mayonnaise; stir to combine.
Add the Swiss cheese and stir to combine.
Pour the mixture into an 8" x 8" baking dish (one which can also be used for serving the dip).
Place in the pre-heated oven and bake for 25 minutes or the top is golden brown and bubbly around the edges.
Remove the dip from the oven and, using a paper towel, gently blot excess oil from the top.
Serve with the toasted French baquette slices.
