Place the 3 large boneless skinless chicken breasts in the slow cooker; sprinkle 1 Tbsp minced garlic and 1½ tsp Italian seasoning over the chicken, then pour the 24-oz jar of spaghetti sauce on top.
Cover and cook on LOW for 6–8 hours (about 360 minutes) until the chicken is fully cooked and tender.
Remove the chicken to a cutting board or bowl and shred with two forks; return shredded chicken to the sauce and stir to combine.
Preheat the oven broiler. Slice the slider rolls in half and place the cut sides up on a baking sheet; broil until edges are golden brown, watching closely.
Preheat the oven to 350°F (177°C). Arrange the bottom halves of the toasted buns in a 9x13-inch baking dish or on a rimmed baking sheet.
Sprinkle a layer of shredded mozzarella over the bottom buns, top with a generous portion of the shredded chicken in marinara, then add more mozzarella and place the top buns over each slider.
In a small saucepan over medium heat, melt 1/2 cup butter, then stir in 1 tsp garlic powder and 1 tsp Italian seasoning; remove from heat and mix in 1/3 cup grated parmesan cheese.
Brush the butter-parmesan mixture over the top buns, then bake the assembled sliders in the preheated 350°F oven until the tops are toasty and the cheese is melted, about 10–15 minutes.
Optionally spoon extra marinara sauce over the sandwiches before serving.
