Preheat oven to 350°. Arrange pecans in a single layer on a baking sheet, and bake at 350° for 5 to 7 minutes or until lightly toasted and fragrant. Cool on a wire rack 15 minutes or until completely cool.
Combine watermelon and mâche in a large bowl; add vinaigrette, tossing gently to coat. Transfer watermelon mixture to a serving platter, and sprinkle evenly with pecans and cheese.