Sizzling Onion Fajitas

1 (1 1/2 pound) top round or flank steak
Marinade
2 medium (8 to 10 ounce each) sweet Spanish onions
2 medium green peppers
1/2 teaspoon lemon pepper
Seasonings
Corn or flour tortillas
Hot pepper-onion relish
Guacamole
1/4 cup butter or margarine

NOTE: Boned chicken breasts may be used in place of beef.

Place steak in shallow dish and cover with marinade. Refrigerate 4 to 6 hours, turning occasionally.

Peel onions and slice thin to measure 5 to 6 cups onion rings. Cut green peppers into julienne strips. Saut