In a large, clean bowl, whisk together the milk, potato flakes, parsley, garlic, salt, thyme and pepper.
Break up the ground sirloin into small pieces and add to the bowl. Use a fork and blend the beef with the milk mixture until combined.
Use a 3 tablespoon capacity cookie scoop to divide the beef mixture into 12 meatballs. Place the meatballs into a single layer on the bottom of the pre-heating slow cooker. You can stack the meatballs if you have to, but it's better if they can be in one layer.
Sauce
In a large, non-stick skillet, melt the butter over medium heat.
Add the pearl onions, carrots and garlic and cook for a couple of minutes, stirring frequently.
Add the red wine. Bring to a boil and allow to cook for a couple of minutes.
Reduce the heat and stir in the crushed tomatoes, sugar and salt.
In a small bowl, whisk together the cornstarch and water until smooth. Stir the mixture into the sauce and allow it to simmer for an additional minute.
Carefully pour the sauce over the meatballs. Do not stir.
Put the lid on the slow cooker and cook on HIGH for 2 hours. After 2 hours, gently stir the meatballs and sauce. Check the temperature of the meatballs to make sure they are 165 degrees F. Turn the heat to LOW until ready to serve.