For the meatballs: In a large bowl combine the ground meats and add the flour, oats, mustard, egg, and salt and pepper. Incorporate all the ingredients using your hands. Form meatballs the size of golf balls.
In a large skillet, heat some oil on medium heat, and fry the meatballs until they are browned on all sides.
Remove the meatballs from the heat and set aside.
For the stew: In the same skillet with all the drippings, saute all the vegetables on medium heat for 5 minutes.
Add the tomato paste and the beef stock, and stir with a spatula to incorporate the flavors and dissolve the paste. To amp the flavor of the sauce, you can add 1/2 cup of dry red wine along with the stock. Cook for 5 more minutes.
Add the meatballs back to the pan, along with any juices. Cook for another 5-7 minutes to incorporate the flavors. Taste the sauce and adjust for seasoning with salt and pepper if needed.
Serve warm with rice and fried green plantains.