Brush both sides of bread slices lightly with melted butter. Cook bread slices in a large skillet over medium-high heat 1 to 2 minutes on each side or until toasted.
Cut tomatoes into 1/4-inch-thick slices, and place in a single layer on a wire rack in a jelly-roll pan. Sprinkle generously with salt and pepper, and let stand 10 minutes.
Process basil, oregano, and 2 Tbsp. oil in a food processor or mini chopper until finely ground. Add mayonnaise and 1 Tbsp. lemon juice to processor; pulse until smooth.
Stir together remaining 1 Tbsp. oil and 1 1/2 tsp. lemon juice in a medium bowl; add arugula, and toss to coat. Spread mayonnaise mixture on 1 side of each bread slice. Layer 4 bread slices with tomato slices, arugula mixture, and 2 bacon slices. Top with remaining 4 bread slices, mayonnaise-side down.