PREHEAT oven to 425°F. Cook and stir peppers and onion in butter in large skillet on medium-high heat until crisp-tender. Stir in cumin. Drain, reserving liquid.
BEAT cream cheese and cheddar cheese with electric mixer on medium speed until well blended. Spoon 2 Tbsp. cheese mixture onto each tortilla; top each evenly with pepper mixture. Fold tortillas in half; place on ungreased baking sheet. Brush with reserved liquid.
BAKE 10 minutes or until heated through. Cut each tortilla into thirds. Serve warm with salsa.