Preheat your oven to 400°F (200°C). Cut the tops off the heads of garlic to expose the cloves. Drizzle with olive oil and wrap each head in aluminum foil. Place them on a roasting pan and bake for about 30-35 minutes, or until the cloves are soft and caramelized. Once done, let them cool slightly before squeezing the roasted garlic into a bowl.
In the bowl with the roasted garlic, add 6 tablespoons of unsalted butter, 2 tablespoons of grated parmesan cheese, 1 tablespoon of chopped fresh herbs, and a pinch of kosher salt. Mix everything together until well combined.
Spread a generous layer of the garlic mixture on one side of each slice of sourdough bread. On the unspread side, pile on the freshly grated fontina cheese, making sure to cover the entire slice. Top with another slice of bread, garlic side out.
Heat a skillet over medium heat. Once hot, place the assembled sandwich in the skillet. Cook for about 3-4 minutes on one side, or until golden brown and crispy. Carefully flip the sandwich and cook for another 3-4 minutes on the other side until the cheese is melted and the bread is perfectly toasted.
Once both sides are golden and the cheese is deliciously melted, remove the sandwich from the skillet. Let it rest for a minute before slicing it in half. Serve warm.