Add chopped bacon into a cold 8-inch, nonstick pan. Turn heat to medium. Cook approximately 10 minutes, stirring occasionally and adjusting heat as needed, or until bacon is browned and crisp. Drain bacon fat, reserving one tablespoon in the pan.
Add shallots to the pan. Season with salt. Sauté about 6-8 minutes or until translucent and sweet.
Add brussels leaves to pan, followed by the butter. Season with salt and pepper. Sauté about 4 minutes or until tender with a slight bite. Remove bacon and vegetables from pan and allow to cool at least 10 minutes before adding to eggs.
Wipe out the 8-inch pan. Warm over medium heat.
In a large bowl, beat eggs with milk and cheese. Season with salt. Stir in bacon and brussels mixture.
When pan is heated, add oil. It should shimmer. Swirl to coat pan.
Pour in egg mixture in the center of the pan. Using a rubber spatula, stir eggs, running the spatula along the edges and across the bottom of the pan as the eggs set up.
Once the eggs are about 50-60% set, after about 2 minutes, stop stirring. Allow the bottom of the frittata to brown, about 2 minutes.
Loosen frittata from the sides of the pan. Using a flat lid of a pan or large plate, place the lid against the top of the pan and invert the frittata onto the lid. Slide frittata back into the pan and cook it on the opposite side.
Cook about another 2 minutes or until browned and just set. Remove from pan.
For the salad dressing, whisk together the mustard and vinegar in a bowl. Slowly drizzle in oil, whisking constantly, until emulsified.
Add frisee and radishes to a bowl. Season lightly with salt. Toss with dressing.
Serve frittata with salad. Enjoy.