Preheat your oven to 400°F (200°C) to prepare for baking the meatballs.
In a large mixing bowl, combine the ground chicken, lean ground chicken breast, salt, pepper, slightly beaten egg, panko bread crumbs, minced garlic, and the diced jalapeño. Mix until just combined without overmixing.
Shape the mixture into meatballs about 1 to 1.5 inches in diameter and place them on a baking sheet lined with foil or parchment paper.
Bake the meatballs for 20-25 minutes until cooked through and reaching an internal temperature of 165°F (75°C).
Remove the meatballs from the oven, sprinkle freshly grated cheddar cheese on top, and return to the oven for 5 minutes until the cheese is melted and bubbly.
Slice the whole wheat sandwich buns and place a few meatballs on the bottom half. Top with thinly sliced jalapeños for extra heat if desired, then close the sandwiches with the top halves.
Serve the sandwiches immediately while warm and cheesy, paired with your favorite sides.