Preheat broiler. Cut all the bell peppers in half lengthwise and remove the stems, seeds and ribs. Place cut sides down on a baking sheet. Broil until the skins blacken and blister. Remove the baking sheet from the oven, drape the peppers loosely with aluminum foil and let cool for 10 minutes. Using your fingers or a small knife, remove the pepper skins. Cut the peppers lengthwise into strips 1/4-inch wide.
In a bowl, combine the bell pepper strips, garlic, vinegar, chervil and salt and pepper to taste. Let marinate unrefrigerated 30 minutes.
In another bowl, whisk together the eggs, milk and Parmesan until frothy. Add the pepper mixture and mix well. Season to taste with salt and pepper.
Preheat oven to 400 degrees. In a 10-inch ovenproof frying pan over medium heat, warm the olive oil. When the oil is hot, add the egg mixture and, when it starts to set, lift the edges of the frittata with a spatula so that some of the egg mixture runs underneath.
Reduce the heat to medium and cook until the bottom is set but the top still is runny, 8 to 10 minutes. Place the pan in the oven and cook until the eggs are set on top and golden brown on the bottom, 6 to 7 minutes.
Remove the frittata from the oven and loosen with a spatula. Invert the frittata onto a serving plate. Garnish with fresh chervil sprigs.