1. Preheat broiler to high.
2. Cut each bread slice into a 3-inch circle using a round cutter; reserve scraps for another use (such as breadcrumbs or croutons). Lightly coat bread rounds with cooking spray. Arrange bread rounds in a single layer on a baking sheet. Broil 2 minutes on each side or until lightly toasted. Cool on a wire rack.
3. Reduce oven temperature to 400°. Place a baking sheet in oven to preheat.
4. Combine rice and next 7 ingredients (through pepper) in a medium bowl. Combine mustard and egg, stirring well. Add egg mixture to rice mixture; stir until well blended. Spoon 1/3 cup rice mixture into a (2 1/2-inch) round cookie cutter; pack mixture down. Remove mold. Repeat procedure 7 times to form 8 patties.
5. Heat a large skillet over medium-high heat. Add 1 tablespoon oil to pan; swirl to coat. Carefully add 4 patties to pan; cook 2 minutes. Carefully transfer patties to preheated baking sheet, turning patties over and arranging in a single layer. Repeat procedure with remaining 1 tablespoon oil and remaining 4 patties. Place pan in oven; bake patties at 400° for 9 minutes. Top each patty with 1 cheese slice; bake an additional 1 minute or until cheese is soft and patties are set.
6. Place 8 toasted bread rounds on a flat surface; top each round with 1 patty. Divide watercress evenly among sliders; top with the remaining toasted bread rounds.