Melt butter in a large saucepan over medium heat.

Add olive oil to the saucepan.

Stir in the thinly sliced onions and salt.

Cook the onions uncovered, stirring occasionally, for 20 to 30 minutes, or until they are golden brown and caramelized.

Sprinkle flour over the caramelized onions and cook, stirring constantly, for 2 to 3 minutes to create a roux.

Remove the saucepan from the heat.

In a separate pan, heat the beef stock to a gentle simmer.

Pour the simmering beef stock into the saucepan with the onions and roux.

Simmer the soup, partially covered, for 30 to 40 minutes to allow the flavors to meld.

While the soup simmers, prepare the croutons.

Preheat your oven to 325°F (160°C).

Place the 1-inch thick slices of French bread on a cookie sheet.

Drizzle the bread slices with 2 tsp of olive oil.

Bake the bread slices in the preheated oven for 15 minutes, or until they are lightly toasted and crispy.

Ladle the French Onion Soup into oven-safe bowls.

Top each bowl with a slice of toasted French bread crouton.

Generously sprinkle grated Swiss cheese (or a combination of Swiss and Parmesan cheese) over the bread and soup.

Place the bowls under a broiler until the cheese is melted and bubbly.