1. Preheat oven to 400°. Drizzle bread with oil; sprinkle with 1/2 tsp. each kosher salt and pepper. Bake bread in a single layer in a jelly-roll pan 12 minutes or until golden.
2. Cook bacon in a skillet over medium heat, stirring occasionally, 10 minutes or until crisp. Drain on paper towels; reserve 1 Tbsp. drippings in skillet.
3. Sauté onion in hot drippings over medium-low heat 3 to 5 minutes or until tender.
4. Smash garlic to make a paste. Whisk together mayonnaise, lemon juice, garlic paste, and remaining 1/2 tsp. each salt and pepper.
5. Toss together tomatoes, arugula, bacon, onion, and salt and pepper to taste in a large bowl. Pour mayonnaise mixture over tomato mixture, and toss to coat. Serve immediately with toasted bread.