Place sliced cucumber in a pitcher with water and refrigerate to infuse. Serve over ice cubes.

Heat olive oil in a heavy-bottomed pot over medium heat.

Add onions and cook, stirring often, until softened, about 10 minutes.

Increase heat to medium-high and cook for about 20 minutes, stirring frequently, until onions begin to brown.

Season with salt and continue cooking, stirring often, until deeply caramelized.

Add garlic and cook for 1 minute until fragrant.

Deglaze with Chardonnay wine, scraping up browned bits.

Add beef broth, bay leaf, thyme, and black pepper. Bring to a gentle simmer, then cover, reduce heat to low, and simmer for about 10 minutes. Remove and discard bay leaf.

Stir in brandy.

Preheat the broiler to low.

Place sourdough bread on a sheet pan and broil until lightly browned, about 5–7 minutes.

Flip bread, top with Gruyère cheese, and broil until melted, bubbly, and lightly browned.

Assemble sandwich by layering deli turkey breast, cheese, romaine lettuce, and tomato between sourdough bread slices.

Serve the soup topped with melted Gruyère crostini, alongside turkey and cheese sandwich and cucumber water.