Preheat oven to 350°F. Melt butter with garlic in an 8" cast-iron skillet over medium heat. Continue to cook, stirring occasionally, until garlic is fragrant and starting to brown, about 4 minutes. Pour into a large bowl, leaving 1 Tbsp. butter in skillet; reserve skillet.
Add eggs, corn, cornmeal, sour cream, baking powder, pepper, and salt to garlic butter and whisk to combine, then mix in Fontina. Scrape into reserved skillet and bake spoonbread until top is golden brown and center is just barely set, 25–30 minutes. Let cool 10 minutes before serving.