Preheat the oven to 425 degrees . Rub a 9-by-13-inch baking dish with the garlic clove and 1 tbsp. butter. In a large pot of boiling, salted water, cook the vegetables until firm-tender, about 7 minutes. Drain and run under cold water to cool.
In a medium bowl, stir the cream, milk, fontina, parmesan, thyme, 1 tsp. salt and 1/4 tsp. pepper.
In a large skillet, melt the remaining 4 tbsp. butter over medium heat. Add the panko and cook, stirring, until golden, about 5 minutes.
Layer the vegetables, cream mixture and breadcrumbs in the baking dish. Bake until bubbling and very tender, about 45 minutes.