In 12-inch skillet over medium heat, in 1tablespoon olive oil, cook onion until just tender. Add spinach; cook 3 minutes until wilted, stirring occasionally. Remove vegetables to bowl; stir in ¾ cup shredded Queso Blanco.
With knife, cut a horizontal pocket in each chicken breast. Spoon spinach mixture into each pocket. In small cup combine salt, cumin and pepper; sprinkle chicken breasts with mixture. With toothpick, skewer each opening of chicken closed.
Preheat oven to 425°F. In 12-inch skillet over medium-high heat, in remaining tablespoon oil, cook chicken breasts until browned on both sides. Remove to plate. Sprinkle top of chicken with remaining ¼ cup Queso Blanco (1 teaspoon on each).Wrap each chicken breast with a slice of prosciutto. Place on baking sheet. Bake 15 to 20 minutes until chicken is tender and prosciutto is browned. Serve with green beans and tostones if desired.