Preheat an oven to 400°F. Arrange the bacon slices on a foil-lined baking sheet and roast until crispy, 12–15 minutes. When the bacon is cooked to your liking, remove it from the pan and drain on paper towels (or use a brown paper grocery bag from the recycling).
While the bacon cooks, slice the cucumbers ¼-inch thick using a mandoline, mince the shallot, and chop the tarragon. Combine in a mixing bowl with the vinegar, sugar, salt, and pepper. Set aside to “quick pickle.”
Peel and mince the garlic or grate it on a Microplane into a small bowl. Stir in the mayonnaise and lemon juice. Season with salt and pepper.
Toast the bread, smear with the aioli, and layer with bacon, quick pickles, and spicy greens. Cut in half and serve.