Preheat the oven to 400°F, and place a wire cooling rack in a rimmed baking sheet.
Lay the bacon on the wire rack, and bake for 15 to 20 minutes, until the bacon is crisp (it will crisp up a bit more as it cools, so take it out when it still has some pliability). Alternately you can cooke the bacon in a skillet.
In a small bowl, combine the mayonnaise, tarragon, lemon juice, and salt and pepper. Spread a thin layer of the mayonnaise mixture on one side of each of the slices of bread. (you should use less than half the mayo mixture). Heat a large skillet over medium heat. Place as many of the bread slices in the skillet as will fit, mayo side down. Allow to cook until very lightly brown, about 2 minutes, then remove from the skillet and repeat with the remaining slices. Set aside.
Stir the minced garlic into the rest of the mayo mixture. Divide the remaining mayo mixture over the bread slices, on the untoasted side. Layer the tomato, avocado, 2 romaine lettuce halves, bacon, and a generous pinch of sprouts on top of the bread. Top with the remaining slices of bread, toasted side up. Cut in half, and serve.
Notes
Buy good bacon. Use good mayo. Find a great tomato. Make sure your avocado is ripe. After that, all you have to do is assemble, and you will have a fantastic sandwich.