In a skillet, cook bacon over medium heat until crisp, stirring occasionally. Using a slotted spoon, remove bacon to paper towels. Discard all but 1 tablespoon drippings.
Add vinegar, garlic, brown sugar, mustard and seasonings to drippings; heat through, stirring to blend. Transfer to a small bowl; gradually whisk in oil. Stir in half of the bacon.
Place spinach, onion and eggs in a large bowl; toss with warm dressing. Sprinkle with remaining bacon; serve immediately.