How to Make It
Make the pistachio butter: Process the pistachios in a food processor until they form a thick paste. With the motor running, drizzle in ⅓ cup of water, the vinegar and salt, followed by the oil. Process until smooth and creamy, scraping down the sides as needed. Taste and adjust the seasoning; the pistachio butter will be a bit thin now, but it will thicken as it stands. It will keep in the refrigerator for weeks, maybe more, but the flavor is best within the first week or so.
Make the toasts: Prepare a very hot charcoal or gas grill, or preheat a grill pan over high heat until smoking. Stand the pear up on a cutting board and trim away two of the bulbous sides, then cut it lengthwise into planks, each about ⅓ inch thick (you should be able to get 4 thick, pear-shaped slices). Trim out the center core from the slices that have it.
Brush the pear and bread slices lightly with oil and sprinkle with flaky salt. Grill the pear pieces and bread, in batches if needed, until deep grill marks appear on both sides, 1 to 2 minutes per side.
Remove the toasts and pear from the grill. Spread about 2 tablespoons of pistachio butter on each piece of toast. (Save the rest of the pistachio butter for another use.) Place a grilled pear slice on top of the pistachio butter layer on each toast. Toss the greens with a light drizzle of oil—just enough to barely coat them—then pile a mound on top of each toast. Sprinkle with flaky salt and serve.