In a large bowl, combine the ground chicken, eggs, onion, garlic, oregano, salt, red pepper flakes, pesto, parmesan, and breadcrumbs. Mix gently until everything is evenly combined. Avoid overmixing because it can make the meatballs dense.
Lightly dampen your hands and roll the mixture into evenly sized meatballs. Making them the same size helps them cook evenly. Arrange them on a plate while you prepare the skillet.
Heat olive oil in a large skillet over medium-high heat. Add the meatballs and let them brown on all sides. This creates a delicious crust and locks in moisture.
Remove the meatballs temporarily and add olive oil to the same skillet. Cook the onions until softened, then stir in garlic, oregano, and red pepper flakes. This creates a flavorful foundation for the sauce.
Pour in the marinara sauce and chicken broth, stirring well. Return the browned meatballs to the skillet and nestle them into the sauce. Cover and simmer until the meatballs are fully cooked and tender.
Place the bread slices or rolls on a baking sheet. Toast them lightly under the broiler until golden. This step helps prevent the sandwiches from becoming soggy.
Top each piece of toasted bread with several meatballs and a generous spoonful of sauce. Sprinkle parmesan cheese over the top and add slices of provolone or goat cheddar.
Place the assembled subs under the broiler for a few minutes. Watch closely as the cheese melts and becomes bubbly and lightly golden.
Combine romaine, arugula, olive oil, vinegar, pepperoncini, oregano, basil, salt, and pepper in a bowl. Toss everything together until evenly coated.
Place a generous amount of fresh greens on top of each meatball sub. Serve immediately while the cheese is still melted and gooey.