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1
Combine the olive oil and onions in a medium Dutch oven and cook over medium heat, stirring occasionally, for 5 minutes.
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2
Add the leeks and garlic, then cover and cook, stirring occasionally, until the leeks are tender, about 15 minutes.
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3
Pour in the wine and cook, uncovered, until the alcohol has evaporated, about 15 minutes.
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4
Add the broth and the bay leaves and bring to a simmer; continue simmering for 10 minutes.
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5
Remove the bay leaves.
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6
Puree 2 cups of the soup in a blender and then stir it back into the pot.
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7
In a large nonstick skillet, cook the prosciutto over medium-high heat, stirring, just until brown; transfer the prosciutto to the soup and reserve the skillet.
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8
Melt 2 tablespoons of the butter in the same skillet over medium heat.
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9
Add half of the baguette slices to the skillet and cook until golden and toasted on the bottom, about 4 minutes.
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10
Turn the baguette slices over and spoon a heaping tablespoon of the shredded cheese on top of each; cover with a lid and cook until the cheese is melted, 2 to 3 minutes more.
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11
Transfer the croutons to a heatproof plate and keep warm in a low oven; repeat with the remaining butter, bread and cheese.
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12
Season the soup with salt and pepper, add the chives and ladle into mugs.
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13
Place a crouton on top of each mug and another on the side and serve.
