Combine salmon, mayonnaise, basil, and tomatoes in a small bowl. Mix well.
Cook the bacon in a large skillet on medium-high heat until crisp. With tongs, transfer the bacon onto paper towels to drain excess oil. Add onions to the same skillet and sauté till it becomes tender and begins to brown (it should take about 5 minutes).
Add the bacon and onions to the salmon mixture and blend evenly.
Spread this mixture evenly onto two slices of bread. Top with a slice of tomato and lettuce leaf.
Stack the slices on top of one other (face up)and top with the third slice of bread.
Secure the slices using toothpicks. Cut into triangles.