Heat the oven to 350 degrees. In a medium sauté pan, heat 1 tablespoon of olive oil over medium heat. Add the onion and a large pinch of salt and cook, stirring frequently, until the onion has begun to soften and lightly caramelize, about 5 minutes. Add the minced garlic and continue to cook until both the onion and garlic are soft and golden brown, about another 5 minutes. Set aside to cool.
In a large bowl, combine ground beef, mayonnaise, dijon mustard and bread crumbs. Gently stir to combine, but do not overmix. Using your hands, work in the egg and the browned onions and garlic, until just combined.
Shape the mixture into 1 1/2-inch meatballs. Place on a rimmed baking sheet, spacing 2" apart. Bake meatballs until lightly browned on top and cooked through, 20–25 minutes.
Meanwhile, heat 1 tablespoon oil in a large skillet over medium-high. Add sliced garlic and cook until golden; spoon out the garlic and set aside. Add the remaining oil and the radishes and cook, tossing occasionally, until golden and crisp-tender, 5–7 minutes. Add kale and cook until just wilted. Stir the garlic back in and remove pan from heat. Remove meatballs from oven and add to the pan with kale and radishes along with their juices. Toss to combine and serve.