COMBINE one-third of bread cubes with milk in a small bowl. Let stand until bread is soft, 5 min.
COMBINE ground beef with 1/4 cup finely chopped parsley, egg yolk, salt and milk-soaked bread cubes in a large bowl. Mix with your hands until combined. Divide mixture and roll into 16 balls.
HEAT an extra-large non-stick frying pan over medium. Add 2 tsp canola oil, then remaining dry bread cubes. Cook, stirring often, until toasted, 4 to 5 min. Transfer to a plate. Add 1 tsp canola oil to pan, then meatballs. Cook, turning often, until brown on all sides, about 5 min. Reduce heat to medium-low and continue cooking, covered, stirring occasionally, until meatballs are cooked through, 7 to 10 min. Let stand, 5 min.
WHISK remaining 2 tbsp each canola oil with cider vinegar and mayo mustard in a large bowl. Add kale and grated beet. Toss to coat. Divide salad and croutons among 4 plates. Top with meatballs. Sprinkle with remaining 2 tbsp chopped parsley.