For the Salsa:
If watermelon is very juicy, place in a strainer over a bowl and allow to drain for a half hour or so. Mix all the ingredients together. Best if sits for about 30 minutes before serving.
For Crostini:
Cut bread thinly on the diagonal. If desired, brush with olive oil or a neutral oil. (If spreading crostini with ricotta, use olive oil, if spreading with mascarpone, use a neutral oil.) Crostini may be toasted on the grill, on a hot skillet for in the oven. If using an oven, move oven rack to about 5 inches below the broiler and place crostini directly on the rack. Broil until lightly browned, then turn crostini over and brown the other side. Watch closely with all methods to prevent burning.
To assemble, spread cooled crostini with ricotta or mascarpone cheese. Sprinkle with Watermelon Salsa. Sprinkle with salt and pepper. Serve shortly after assembly.