Halve the chicken breasts crosswise and pound each piece between two sheets of plastic wrap to an even thickness of about 3/4 inch. Toss the chicken pieces with the crushed red pepper, fennel seed, 1 tablespoon of the oil and 3/4 teaspoon salt in a medium bowl. Brush the insides of the rolls with the remaining 1 tablespoon oil.
Grill the chicken, turning once, until just cooked through, 8 to 9 minutes per side. During the last few minutes of cooking time, top each piece of chicken with a slice of mozzarella. Grill the rolls, oiled side down, until warmed and lightly charred, about 1 minute.
Spread some pesto on the bottom halves of the rolls. Top with a slice of tomato, a piece of chicken, the remaining pesto and the top halves of the rolls.
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