To make the onion confit: heat the oil and butter in a large sauté pan over low heat. Add the onions and cook, stirring occasionally, until the onions are soft and translucent, about 30 minutes. Add the vinegar and honey and stir to coat. Season with salt and pepper, to taste. Continue cooking, stirring occasionally, until most of the liquid has evaporated and the onions are nicely browned, about 45 minutes.
The onion confit makes more than you will need for the sandwiches. The leftovers will keep, refrigerated, in an airtight container for up to five days. It can be used as a topping for steaks or pizzas or added to soup or pasta dishes. The confit can be made ahead of time; just reheat in a saucepan and serve.
(makes approximately 1 cup)
Micro Lemon Basil Mayonnaise:
To make the micro lemon basil mayonnaise: in a small bowl, mix together the mayonnaise, basil, and lemon zest. Cover and refrigerate until ready to use.
To make the bacon: preheat the oven to 400º. Lay the bacon in a single layer on a foil-lined, rimmed baking sheet. Place the bacon in the oven and bake for 6 minutes then rotate the baking sheet and continue to bake until the bacon is golden brown and crisp, about 7 minutes, or until desired texture and color is achieved. Using tongs, carefully transfer the bacon to a large plate lined with paper towels to drain. While the bacon is draining, slice the tomatoes into ¼-inch-thick slices. Drizzle with olive oil and season with salt and pepper to taste. Let stand for 5 minutes.
While the bacon is draining, toast the bread. Place the bread slices on a baking sheet and drizzle with olive oil and bake until lightly toasted, about 5 minutes.
To assemble the Sandwiches:
Spread approximately 1 teaspoon of the mayonnaise onto each bread slice, or to taste. Spread 1 tablespoon of the confit over the 2 bottom slices, or to taste. Divide the lettuce over the 2 bottom slices, top with tomato slices, then bacon. Top with the remaining bread slices. Serve at once.