Caramelize the sliced onions, garlic, shallots, and leeks in olive oil and butter very well until deep brown and sticky. This will take about 2 hours over low heat. Be careful not to keep the heat too high as the onions deepen in color as they will burn more easily.
Add the cumin, thyme, and sherry and stir to deglaze the pot. Add the broth and seasonings and bring to a boil. Simmer for 1 minimum of when hour.
Using French onion soup bowls or another steep sided soup vessel, ladle the broth into the bowl leaving 1/2 inch from the top. Place toasted baguette slices on top and cover with sliced cheese. Broil for 2 minutes or until the cheese is golden and melted. Serve immediately.