Adjust oven rack to middle position and preheat oven to 200°F (95°C). In a large bowl, whisk together eggs, milk, garlic, red pepper flakes, parsley, 1 ounce cheese, and salt and pepper to taste. Place remaining parmesan on a large plate.
Heat 2 tablespoons butter in a large nonstick or cast iron skillet or griddle over medium heat until melted. Working one slice at a time, dip bread in egg mixture, then dredge in grated parmesan to coat well on both sides. Lay in skillet. Repeat until 4 slices of bread are in skillet. Cook, swirling pan and flipping occasionally, until deep golden brown on both sides, about 8 minutes total. Transfer to a large plate and keep warm in the oven. Repeat with remaining butter and second batch of bread.
Sprinkle French toast with scallions and more chopped parsley and serve immediately.
