Trim and thinly slice the yellow, white and red onions. Separate scallion whites and greens; thinly slice both. Trim, halve, and slice the leek, then rinse well to remove grit. Mince garlic.

Sweat onions and leeks

In a large heavy-bottomed pot over medium heat, melt the butter with the olive oil. Add the sliced yellow, white and red onions, the sliced leek, and the scallion whites. Stir to coat, then season lightly with a pinch of salt. Cook, stirring every few minutes, until the onions are softened and beginning to caramelize, about 20–25 minutes. Lower the heat if they start to brown too quickly.

Add garlic and thyme

Add the minced garlic and fresh thyme (or dried). Cook 1–2 minutes until fragrant, stirring constantly so the garlic doesn’t burn.

Deglaze

Pour in 3 tablespoons dry sherry, scraping up any browned bits from the bottom of the pot. Allow the alcohol to mostly evaporate, about 1–2 minutes.

Simmer with stock

Add the bay leaf and pour in 6 cups of stock. Bring to a gentle simmer, reduce heat to low, partially cover, and simmer for 25 minutes to let flavors meld.

Finish soup body

Stir in the whole milk and 1 cup heavy cream. Simmer gently for 5 minutes — do not boil vigorously once dairy is added to avoid curdling. Taste and season with 1 teaspoon salt and 1 teaspoon freshly ground black pepper, adjusting to preference. Remove and discard the bay leaf.

Sherry-port cream finish

Prepare finish

Whisk together 1 tablespoon dry sherry, 1 tablespoon ruby port, 1/2 cup heavy cream, 1/4 teaspoon salt and 1/4 teaspoon pepper in a small bowl.

Incorporate and adjust

Ladle a cup of hot soup into the cream mixture to temper it, then whisk that back into the pot. Warm through over low heat for 2–3 minutes, tasting and adjusting seasoning. Keep warm while you prepare crostini.

Crostini & topping

Toast crostini

Preheat the oven to 400°F (200°C). Arrange baguette slices on a baking sheet and brush both sides lightly with 2 tablespoons olive oil. Toast until golden and crisp, 8–10 minutes, flipping once. Remove from oven and rub one side of each warm crostini with the halved garlic clove.

Assemble and broil

Ladle the soup into oven-safe bowls. Place 1–2 crostini on each bowl (or set crostini on a separate sheet if you prefer topping). Sprinkle a generous layer of grated Gruyère (about 1/3 to 1/2 cup per serving) and a little Parmesan on top of each crostini or directly on the soup surface. Place bowls on a baking sheet and broil in the oven until the cheese is melted, bubbling and lightly browned, 2–4 minutes. Watch closely to prevent burning.

Serve

Carefully remove bowls from oven (they will be hot). If you broiled crostini separately, place them atop the soup now. Finish with a light drizzle of sherry or port (optional) and a sprinkle of chopped chives or sliced scallion greens.