In a small bowl, combine the mayonnaise and minced garlic. Mix well and set aside.
In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add the sliced red onions and sprinkle with sugar. Sauté for about 5-7 minutes until the onions become soft and caramelized.
Stir in the balsamic vinegar and crushed red pepper flakes. Cook for an additional minute, then remove from heat.
Season the chicken breasts with salt, pepper, and dried thyme.
In the same skillet, add the remaining 1/2 tablespoon of olive oil, and increase the heat to medium-high. Add the chicken breasts and cook for about 6-7 minutes on each side, or until the internal temperature reaches 165°F (75°C).
Remove the chicken from the skillet and let it rest for a few minutes before slicing.
Roll out the refrigerated pizza crust on a lightly floured surface to your desired thickness. Cut it into rectangles that will comfortably hold your fillings.
Spread a generous amount of the garlic mayonnaise on one side of each piece of dough.
Layer the chopped spinach, sliced chicken, caramelized onions, and provolone cheese on half of the dough pieces. Top them with the other half, pressing gently to seal.
Preheat your panini press or griddle.
Place the assembled paninis on the press and cook until the bread is golden brown and the cheese has melted, about 3-5 minutes. If using a griddle, flip the paninis halfway through for even cooking.
Once toasted to perfection, remove the paninis from the heat. Cut in half and serve immediately while the cheese is still gooey and delicious.