Using a food processor, grind the bread into coarse crumbs. In a small nonstick skillet, toast the crumbs with the olive oil over medium-low heat, stirring often, for about 5 minutes. Let cool.
In the food processor, pulse the spinach, scallions, mayonnaise and sour cream until almost smooth; season with salt and pepper.
To serve, spoon the spinach mixture onto the endive spears and top with the toasted breadcrumbs.