Heat a grill pan, griddle or large cast-iron skillet over medium-high.
Brush portabello caps with olive oil; season with salt and pepper. Grill the mushrooms until tender, turning once, 6 to 8 minutes. Liberally baste with HP Sauce.
Build sandwiches with the toast, mushrooms, watercress, blue cheese and tomatoes. Cut the sandwiches corner to corner.