Preheat broiler.
Melt butter in a large skillet over medium-high heat. Add onions, bay leaf and thyme, season with salt and pepper, and sauté until onions start to brown, about 10-15 minutes. Lower heat to medium and continue to cook the onions, stirring frequently, until deep golden brown, about 20 minutes. (If the onions begin to dry out, add a little water to the pan.) Deglaze the pan with the sherry or wine, let it evaporate a bit then remove from heat and stir in the crème fraiche.
In another large, shallow skillet, bring the consommé to a simmer over medium-high, just a minute or two.
Cut the rolls in half horizontally, but not all the way through, leaving a hinge at one edge.
Dip each roll in the consommé, turning to coat. Using tongs, dip the roast beef a few slices at a time, into the consommé and fill the rolls. Top with caramelized onions and cheese. Place sandwiches on a baking sheet and set under the broiler until brown and bubbly.
Serve extra consommé alongside for dipping.
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