Finely zest the lemon half. Put the zest, olives, rosemary, and garlic in a food processor. Add the olive oil through the feed tube, pulsing the machine until you get a thick, uniform paste. Transfer to a bowl and stir in a squeeze of lemon juice (about 1 tsp.). Season with salt, pepper, and more lemon juice to taste.
To assemble, spread the tapenade on the toasts, slice each toast into four even triangles, top each with a slice of red pepper, and serve.
Make Ahead Tips
You can make the tapenade up to two days in advance. Refrigerate, covered; bring to room temperature before assembling.
Photo: Scott Phillips