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

While the garlic roasts, boil a pot of water, add the russet potatoes, and cook for about 15–20 minutes until fork-tender. Drain and set aside.

In a large pot or Dutch oven, melt the butter over medium heat. Add diced onions and sliced leeks, stirring for 5–7 minutes until softened.

Lower the heat and stir in the flour, cooking for 1 minute until lightly golden. Whisk in the broth, milk, cream, and thyme, seasoning with salt and pepper.

Add the cooked potatoes and roasted garlic, stir in Parmesan cheese, and let everything bubble together for a few minutes.

Using an immersion blender, purée the soup until smooth. If too thick, add more broth until desired consistency is achieved.

Butter one side of each bread slice, pile shredded Gruyère on the unbuttered side of two slices, and top with the other slices, buttered side out.

Heat a skillet over medium heat and cook sandwiches for about 5 minutes on each side until golden brown and cheese melts.

Remove from skillet, cut into 1-inch croutons, and garnish the soup with these before serving.