Preheat your oven to 375–400°F (190–200°C). Slice the top off a head of garlic, drizzle with olive oil, and season with salt and pepper. Wrap in foil and roast for 40–45 minutes until soft.

In a large pot, bring water to a boil and add russet potatoes. Cook until fork-tender, about 15–20 minutes. Drain and cool before setting aside.

Melt butter in a large pot over medium heat. Add chopped onions and sliced leeks, sautéing for 5–7 minutes until translucent.

Stir in all-purpose flour for about 1 minute. Gradually whisk in broth, milk, and cream until slightly thickened.

Add cooked potatoes and roasted garlic cloves to the pot with grated Parmesan. Puree with an immersion blender until smooth, adding more broth if needed.

Butter one side of each slice of bread. Layer Gruyère cheese between two slices, cooking in a skillet until golden brown, about 5 minutes per side. Cut into croutons.

Serve the soup in bowls, garnished with grated Parmesan, chives, and croutons.