Preheat the oven to 375 degrees F.
Place the tomato slices on a tray or large plate. Sprinkle with fleur de sel and black pepper. Drizzle the vinegar and oil over the tomatoes. Allow to sit for 15 minutes.
Lay the bacon on a baking sheet, and season with a generous amount of black pepper. Brush some of the juices from the seasoned tomatoes onto the bacon and top with the red onions. Roast the bacon until it is golden brown and the fat has rendered, about 20 minutes.
Spread 1 1/2 teaspoons mayonnaise on each slice of country toast. Top each with 3 slices of bacon and the onions. Reserve the bacon drippings. Season the avocado with salt and pepper, and place on top of the bacon. Follow with 2 marinated tomato slices on each sandwich.
In a separate bowl, lightly toss the lettuce and tarragon with a small amount of the drippings from the bacon and season as needed with salt and pepper. Top the sandwiches with the salad and serve warm.