In a large bowl, combine the ground chicken, egg, 1/2 cup grated Parmesan, Panko breadcrumbs, 2 minced garlic cloves, 1 teaspoon Italian seasonings, 1 teaspoon kosher salt, 1/2 teaspoon black pepper, and 1/2 teaspoon crushed red pepper; mix until just combined.
Shape the mixture into about 20 meatballs, roughly 1½ inches each, using a cookie scoop or your hands; if the mixture sticks, dampen your hands with water.
Heat 2 tablespoons olive oil in a large skillet over medium heat. Add the meatballs and brown on all sides, about 2–3 minutes per side; they should be browned but not fully cooked. Remove and keep warm on a plate.
If the pan has breadcrumb residue, wipe it out. Add 1 tablespoon oil from the sun-dried tomato jar and heat over medium; add 2 minced garlic cloves and cook 30 seconds until fragrant.
Stir in 1 tablespoon tomato paste, 1/2 cup chopped sun-dried tomatoes, 1 cup chicken broth, 1 cup heavy cream, 1/2 cup grated Parmesan, and 1 teaspoon dried Italian seasonings; bring the sauce to a simmer.
Return the browned meatballs to the skillet and simmer gently for about 10 minutes, or until meatballs are cooked through and the sauce has thickened; taste and adjust salt and pepper if needed.
Stir in the chopped spinach and torn basil leaves a few minutes before the end of cooking and cook just until the spinach wilts. Serve hot with extra Parmesan if desired.