Beat cream cheese, eggs, salt and seasonings until well blended.
Melt butter in 10-inch ovenproof skillet on medium heat. Add mushrooms and peppers; cook and stir 5 min. or until crisp-tender. Stir in pepperoni and cream cheese mixture. Cover; cook 5 min. or until center is almost set.
Heat broiler. Uncover frittata. Broil, 6 inches from heat, 2 to 3 min. or until golden brown.