Pound the thickest parts of the thighs to an even 1/2 inch between sheets of parchment. Season with 1/2 teaspoon salt and pepper.
Heat olive oil in a saucepan over medium heat and cook garlic for 30 seconds. Add passata, basil and remaining salt and simmer for 15 minutes until thick enough to leave a spoon trail.
Put flour in one shallow dish, beat eggs in a second, and combine crushed cornflakes with Parmesan in a third. Coat chicken lightly in flour, then egg, then the crumb mixture, pressing gently.
Heat neutral oil in a deep skillet to about 325°F. Fry chicken in two batches for 4 to 5 minutes per side, adjusting the heat to keep the coating golden, until the centers reach 165°F; drain on a rack.
Heat the broiler. Split rolls without cutting through the hinge and toast the cut faces for 1 to 2 minutes. Cut each fried thigh into two broad strips.
Arrange two chicken strips along each roll, spoon thick sauce down the center and divide mozzarella over the sauce. Broil for 2 to 3 minutes until cheese melts, keeping the roll edges from burning.
Rest for 2 minutes and close the rolls around the filling. Serve one sub per person while the exposed crumb edges are crisp.