Heat the oven to 425°F and oil a rimmed sheet with the teaspoon oil. Soak panko in milk for 3 minutes, then mix gently with beef, pork, egg, grated onion, meatball salt, paprika, thyme, cayenne and pepper. Form 24 meatballs and bake for 16 to 20 minutes until the centers reach 160°F.
Meanwhile, heat 1 tablespoon gravy oil in a saucepan and cook diced onion, bell pepper and celery for 7 minutes until soft. Transfer vegetables to a bowl.
Add remaining gravy oil and flour to the pan. Cook over medium heat for 5 to 7 minutes, stirring constantly, until peanut-butter brown; gradually whisk in broth, return vegetables and add gravy salt, then simmer for 3 minutes.
Transfer hot gravy and hot meatballs to a 4-quart slow cooker. Cover and cook on HIGH for 2 hours, turning gently once, until meatball centers reach 165°F.
Near serving, bring grits water and grits salt to a boil. Whisk in grits, reduce to low and cook for 5 to 7 minutes, stirring often, until tender; remove from heat and fold in butter and cheddar.
Divide grits among 6 bowls and top each with 4 meatballs and vegetable gravy. Serve while the grits are soft.