Prepare the cucumber water: Place the cucumber slices in a pitcher with water. Refrigerate to infuse. Serve over ice when ready.

In a heavy-bottomed pot, heat olive oil on medium. Add onions and cook, stirring often, until softened, about 10 minutes. Increase heat to medium-high, add butter, and cook ~20 minutes, stirring frequently, until the onions begin to brown. Sprinkle sugar and salt, then continue cooking ~10 minutes until deeply caramelized. Add garlic and cook for 1 minute.

Deglaze the pot with Chardonnay wine, scraping up browned bits. Add beef broth, bay leaf, thyme, and black pepper. Bring to a simmer, cover, and reduce to low for about 10 minutes. Taste, adjust salt and pepper as needed, and discard the bay leaf. Stir in brandy if using.

While the soup simmers, place eggs in a saucepan and cover with cold water. Bring to a boil and cook for 8 minutes. Cool under cold running water, peel, and chop.

Lightly toast the sourdough bread for the sandwiches. For the soup topping, brush the French bread slices with olive oil. Place on a sheet pan under a low broiler and toast until lightly browned, about 5–7 minutes. Turn slices, sprinkle with Gruyere and Parmesan, and broil until the cheese is melted, bubbly, and lightly browned.

Make the egg salad: In a small bowl, mix chopped eggs with mayonnaise, dill, salt, and pepper.

Assemble the sandwich: Layer spinach, tomato, onion, and egg salad on one slice of toasted sourdough. Top with the remaining slice of bread and cut in half.

Serve the French onion soup topped with cheesy crostini on top, with half egg salad sandwich and cucumber water.