1/2 cup all-purpose flour
1/2 tsp salt
1/2 tsp pepper
4 boneless, skinless chicken breasts
4 tsp extra-virgin olive oil, divided
12 oz baby spinach
1 cup marinara sauce
1/4 cup grated Parmesan cheese
1/2 cup shredded mozzarella
4 sandwich rolls, preferrably toasted

Position oven rack in top position; preheat broiler.
Combine flour, salt and pepper in a shallow dish. Place chicken between 2 large pieces of plastic wrap. Pound with the smooth side of a meat mallet until the chicken is an even 1/4-inch thickness. Coat the chicken in the flour mixture.

Add 2 tsp olive oil to the pan. Add half the chicken and cook until golden brown, 1 to 2 minutes per side. Transfer to a large baking sheet. Repeat with the remaining olive oil and chicken; transfer those to the baking sheet too.

Top each piece of chicken with the marinara sauce and Parmesan. Sprinkle with mozzarella. Broil until the cheese is melted and the chicken is cooked through, about 3 minutes. Serve on rolls.