To prepare nuts, place sugar in a small skillet over medium heat; cook 90 seconds or until sugar dissolves, stirring as needed so sugar dissolves evenly and doesn't burn. Reduce heat; stir in walnuts. Cook over low heat 30 seconds or until golden. Spread mixture onto foil coated with cooking spray. Cool completely; break into small pieces.
To prepare dressing, place cider in a small saucepan over medium-high heat; bring to a boil. Cook until reduced to 2 tablespoons (about 5 minutes). Combine water and cornstarch in a small bowl; add to pan. Bring cider mixture to a boil, stirring constantly; cook 30 seconds. Remove from heat. Stir in shallots, vinegar, salt, and pepper; let cool.
To prepare salad, place salad mix in a large bowl. Drizzle with dressing; toss gently to coat. Divide evenly among 4 plates; top with berries, cheese, and walnuts. Serve immediately.