Position a rack in the middle of the oven; preheat to 375 degrees . Coat each ramekin with 1 tsp. butter. Add 1/2 tsp. flour to each, then tilt to coat, shaking out the excess.
Fill a large saucepan with 1 inch of water and bring to a simmer over medium heat. Place the egg yolks in a small bowl and the 4 egg whites in a large metal bowl.
Toss the cheese, the remaining 2 tbsp. flour and a pinch each salt and pepper in a large metal bowl; stir in the milk. Place the bowl over the simmering water; stir until the cheese melts and the mixture is steaming, about 5 minutes.
Add 2 tbsp. of the cheese mixture to the yolks, stirring with a small whisk. Whisk the yolk mixture into the hot cheese mixture and remove from the heat.
Whip the egg whites, using a large whisk or an electric mixer, until stiff but not dry, about 1 minute. Fold one-third of the whites into the cheese mixture, then fold in the remaining whites; mix gently.
Spoon the mixture into the ramekins, filling each three-quarters full. Place on a baking sheet; transfer to the oven. Bake, without opening the oven, until the souffles are puffed, about 15 minutes.
Make the salad while the souffles bake. Mix the grapes and mustard in a salad bowl. Toss with the greens, EVOO and vinegar; season. Top with the nuts.
Remove the souffles from the oven with a spatula; transfer them to plates. Serve immediately with the salad.