How to Make It
On a work surface, using the flat side of a large knife, crush the garlic with 1/4 teaspoon salt until almost pureed. In a small bowl, stir together the garlic, mayonnaise and parsley.
Using a thin sharp knife, cut the block of tempeh crosswise on an angle into 12 slices about 1/4 inch thick. Brush the slices on both sides with the vinegar and season lightly with salt and pepper.
In a large nonstick skillet, cook the onion in 1 teaspoon of the oil over moderate heat, stirring occasionally and separating the onion into rings, until slightly softened, about 5 minutes. Transfer the onion to a plate.
Add 2 teaspoons of the oil to the skillet and swirl the pan to coat. Add half the tempeh in a single layer and cook over moderate heat, pressing lightly with a spatula and turning once, until lightly browned, 1 1/2 to 2 minutes per side. Transfer the tempeh to a plate and repeat with the remaining 1 teaspoon oil and tempeh.
Toast the bread and spread each piece with the prepared mayonnaise. Place the tempeh on 4 pieces of toast and top with the onion rings, tomatoes and arugula. Sandwich together with the other pieces of toast and serve at once.