In a large bowl, combine chicken, cheese, breadcrumbs, egg, cream/milk, Italian seasoning, and garlic. Mix until just combined. Don't overmix.
Shape into approximately 1-inch meatballs. You should have about 15 meatballs, give or take a couple.
In a skillet, heat olive oil over high heat.
Once oil is hot, add meatballs, spacing at least 1" apart. You will probably have to do this in batches (it took me two batches).
Cook for 1 minute, until slightly golden, then flip and cook the other side for another minute. You only really need to do this on two sides.
Place browned meatballs in slow cooker.
Top with marinara sauce then cook on high for 2-3 hours or low for 6 hours.
Once meatballs are finished cooking, preheat broiler.
Place 2-3 meatballs onto each hoagie bun, depending on the size of your buns and your meatballs, then top with mozzarella cheese slices.
Place under broiler for 2-3 minutes, until cheese is golden.
Sprinkle with fresh basil.
Serve immediately, and enjoy!