Delicious Italian Meatball Sliders

Name: Delicious Italian Meatball Sliders
Ingredients: 1 ½ pounds lean ground beef; 4 ounces bulk Italian pork sausage; ¼ cup bread crumbs; 1 small egg; 1 teaspoon Cajun seasoning; 1 teaspoon seasoned salt; ½ teaspoon garlic powder; ½ teaspoon ground black pepper; ½ teaspoon salt; 1 tablespoon olive oil, or as needed; 1 small onion, finely diced; 1 (24 ounce) jar tomato and basil spaghetti sauce (such as Ragu®); 1 teaspoon garlic powder; 12 (3 ounce) ciabatta buns; 3 tablespoons garlic butter, or to taste; ⅓ cup shredded Italian cheese blend, or to taste