Heat oven to 400ºF.
Melt 2 tablespoons butter in a heavy 9-inch skillet (preferably oven-safe) over medium-high heat. Add leeks and cook, stirring occasionally, until softened and golden brown, about 10 minutes. Add chicken stock and cream. Reduce heat and simmer until liquid is reduced by half, 12 to 15 minutes.
Meanwhile, warm oil in another large, heavy skillet over medium-high heat.
Add mushrooms and cook, stirring occasionally, until golden and crisp, about 12 minutes. Lower heat to medium and add garlic and thyme. Cook for 1 minute. Season with salt and pepper.Add mushrooms and fontina to leek mixture and stir. Transfer to a 9-inch-by-9-inch baking dish if skillet is not oven-safe.
In empty skillet, melt remaining 2 tablespoons butter over medium heat. Add breadcrumbs and cook, stirring frequently, until golden and crispy, about 8 minutes. Top mushroom-leek mixture with breadcrumbs.
Bake until golden and bubbling, 15 to 20 minutes.