Preheat oven to 400F.
Spread bacon slices on a baking sheet. Place in oven and bake for 12 minutes. Flip and bake for an additional 5-10 minutes until golden brown and crisp. Remove from oven. Drain on paper towels and set aside.
In a small bowl or pan, melt butter in a microwave or stove top. Brush slices of bread with melted butter. Heat grill pan over medium heat. Grill slices of buttered bread on both sides until toasted.
In a small bowl, mix together mayonnaise and garlic. In another small bowl, toss together arugula and vinaigrette.
To make the sandwiches, spread garlic mayonnaise onto grilled bread. Layer with 3 slices of bacon, avocado, tomatoes, and arugula. Season with salt and pepper.