Heat 1 tablespoon oil in large non-stick over medium heat for about 1 minute. Add onions and peppers. Saute over medium heat, stirring occasionally until softened but not soggy, about 2 minutes. Transfer to bowl. Add remaining 1 tablespoon oil to pan and carefully coat bottom and sides.
While vegetables cook, whisk together eggs and milk in a medium-sized bowl until fully blended, about 1 minute.
Heat prepared pan over medium heat for about 1 minute until oil glistens. Pour egg mixture into coated pan. Sprinkle beans, and onion pepper mixture evenly over egg mixture. Break cheese into smaller pieces, about 1 1/2-inch size to easily melt. Place cheese pieces evenly on top. Cook frittata over medium-low heat until eggs are almost set, about 15 minutes. Remove from heat, cover and let set an additional 5 to 10 minutes to finish cooking from residual heat. Cut into 8 pieces. Serve.