Prepare the polenta: Line an 8 by 4-inch loaf pan with plastic wrap and set aside.
Bring the water to a rolling boil in a large pot. Add the salt, then gradually sprinkle in the cornmeal, stirring constantly to avoid lumps. Keep stirring after the last of the cornmeal has been added: the mixture will begin to thicken quite quickly. Reduce the heat to medium and cook the polenta for about 30 to 40 minutes, stirring frequently. Spoon a little of the polenta onto a plate and taste it: when it is no longer crunchy and granular, it's done.
Scrape the polenta into the loaf pan, flattening the top and pressing the mass into a rough rectangle, ending halfway to two-thirds down the length of the pan. This sounds a little confusing, but you should now have a 2-inch-thick rectangle of polenta pressed into the loaf pan. It will be very stiff and have no trouble staying put. Cool the polenta to room temperature, then press a piece of plastic wrap onto the surface and refrigerate until chilled, at least 2 hours, or up to 3 days.
No more that 2 or 3 hours before you plan to serve the dessert, remove the loaf pan from the refrigerator and unmold the polenta. With a long, thin-bladed knife, slice the loaf into 1/4-inch thick slices and cut each of these in half. You should end up with about 24 (2-inch) squares, 1/4-inch thick.
Heat 2 tablespoons of butter and 1/2 tablespoon of vegetable oil in a large frying pan (non-stick is fine) over medium heat. When the foam subsides, add half of the polenta slices and fry about 4 minutes, turning once, until golden and crisp on both sides. Carefully transfer the slices to a platter lined with paper towels and sprinkle the slices with half of the sugar. Repeat with the remaining butter, oil, polenta, and sugar. Cool the sugared polenta slices to room temperature.
To assemble the crostini, spoon the mascarpone into a piping bag fitted with a medium star tip (about 1/2-inch or so). Pipe a generous amount of mascarpone on each polenta square and arrange two or three raspberries on the cheese. Press some pistachio pieces around the berries into the cheese and drizzle the crostini with a little of the honey (a honey dripper or even a teaspoon dipped into the jar then waved over the squares works really well, don't even bother measuring the honey!). The finished crostini should be eaten as soon as possible, but can be kept covered at room temperature for a few hours.