Set your Ninja Foodi Possible Cooker Pro to SEAR/SAUTÉ on high. Add olive oil, then stir in the diced onion, celery, carrots, and minced garlic. Cook for about 6 minutes, until the vegetables are softened and fragrant.
Add the ground beef directly to the pot, breaking it up with a spoon. Cook for about 5 minutes, or until the meat is browned and fully cooked.
Stir in the tomato paste and cook for 1–2 minutes to bring out its deep, concentrated flavor.
Pour in the red wine and stir well, scraping any browned bits from the bottom of the pot. Let the wine reduce slightly, about 3 minutes.
Break up the whole peeled tomatoes with a spoon and add them to the pot along with the milk. Stir until everything is well combined.
Close the lid and switch the cooker to SLOW COOK on high for 4 hours. Let the sauce simmer and develop rich, deep flavors.
Once cooking is complete, remove the lid and season the sauce with salt and pepper to taste. Serve over your favorite pasta, or store in the fridge for up to one week.