For the tomatoes: Preheat the oven to 400 degrees F.
On a small rimmed baking sheet, spread the tomatoes, olive oil, salt, garlic and basil. Toss well to coat. Roast until the tomatoes are popped and beginning to brown, about 25 minutes. Allow to cool slightly.
For the spread: Mix together the mayonnaise and mustard in a small bowl.
For the sandwich: Lay out the bread and spread the mayonnaise-mustard mixture on each slice. Working 2 slices at a time, place 2 lettuce leaves on one slice and about 6 roasted tomatoes on the opposite slice. Ribbon 3 slices of turkey on the lettuce. Place the 2 halves together and secure with 2 toothpicks with a cornichon on each one. Continue with the remaining ingredients. Cut in half before serving.