Bake walnuts in an 8- or 9-inch pan in a 400° oven until golden, 5 to 8 minutes (about 4 minutes in a convection oven).
In a large bowl, combine oil and lemon juice. Add salad mix and stir, adding salt and pepper to taste. With 2 large spoons, divide salad equally among plates.
Cut cheese into thin slices and divide evenly among salads. Sprinkle evenly with walnuts.