Preheat the oven to 375 degrees F (190 degrees C).
Make the meatballs: Mix beef, onion, cooked rice, seasoned salt, garlic salt, bread crumbs, and evaporated milk together in a bowl until well combined. Shape into 1-inch meatballs. Place meatballs onto a rimmed baking sheet.
Bake in the preheated oven until brown, about 15 minutes.
While the meatballs are cooking, make the sauce: Melt butter in a skillet over medium heat. Add mushrooms and sprinkle with a pinch of salt; sauté until mushrooms release their moisture and soften, about 10 minutes. Add wine, water, condensed soup, 1/2 teaspoon salt, and sage; stir to combine.
Transfer meatballs to the sauce and simmer until cooked through and hot, 30 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (71 degrees C).
Meanwhile, prepare mashed potatoes according to the package instructions. Serve with meatballs and mushroom sauce.