Place chicken between 2 sheets of heavy-duty plastic wrap, and flatten to 1/8-inch thickness, using a meat mallet or rolling pin. Sprinkle evenly with salt and pepper, and set aside.
Sauté spinach and onion in hot oil in a large skillet over medium-high heat 3 to 4 minutes or until onion is tender. Remove from heat, and stir in cream cheese until blended. Stir in mozzarella cheese and next 6 ingredients.
Spoon 1/4 cup spinach mixture on center of each chicken breast half, and roll up, jellyroll fashion.
Unfold phyllo sheets on a lightly floured surface. Stack 2 phyllo sheets, brushing with melted butter between sheets. (Keep remaining phyllo sheets covered with plastic wrap to prevent drying out.) Place 1 chicken roll on short side of phyllo stack; gently roll up, folding in long side. Repeat procedure with remaining pastry, melted butter, and chicken. Place rolls in a shallow pan, and brush with melted butter.
Bake at 350° for 35 to 40 minutes or until done. Serve on a bed of mixed greens, if desired.