Combine salmon, Greek yogurt, celery, mustard, lemon juice, garlic powder, salt, pepper, and paprika in a medium bowl.
Preheat an outdoor grill for medium heat and lightly oil the grate. Grill 1 onion half until tender and beginning to caramelize, 5 to 7 minutes per side. Remove from grill; slice when cool enough to handle.
Spread salmon salad over each slice of bread. Top with grilled onions, avocado, and baby spinach.