1. Heat 1 tablespoon of the olive oil in a 12-inch skillet over medium heat. Add onions and cook, stirring often until golden brown and caramelized, about 15 to 25 minutes. Transfer onions to a small bowl, cover and keep warm. Add bacon to skillet and cook over medium-high heat until crisp, 5 minutes. Transfer with a slotted spoon to a paper towel and drain. Crumble bacon and set aside. Discard bacon fat from skillet.
2. Meanwhile, in a large saucepan place chicken, 1 teaspoon salt, and enough water to cover by 1 inch. Bring to a boil. Reduce heat to low and simmer until chicken is just cooked through, 5 to 7 minutes. Drain. When cool enough to handle, cut chicken into slices and set aside.
3. In a small microwave proof bowl, combine remaining olive oil, remaining salt, vinegar, mustard, thyme, and pepper. Heat in microwave on High 1 minute. In large bowl, pour hot vinaigrette over spinach, onions, and tomatoes and toss well to coat. Transfer mixture to a serving platter and arrange chicken and bacon over top. Serve immediately. Makes 4 to 6 servings.