Place the Boneless, Skinless Chicken Breast (2) into a gallon-size Ziploc bag. Using a meat mallet, a rolling pin, or a wine bottle, pound the chicken breasts through the plastic until they are 1/2 inch thick.
In a small measuring cup, whisk together the Lemon (1), 3 tablespoons Extra-Virgin Olive Oil (to taste), Kosher Salt and Freshly Ground Black Pepper (to taste) . Pour marinade into the bag with the chicken, seal, and refrigerate for 1 hour.
Meanwhile, in a large bowl, combine the Baby Arugula (2 1/2 cup), Radicchio (1/2), Walnut (1/2), and Red Seedless Grapes (2/3 cup).
In a small measuring cup, whisk together the Sherry Vinegar (1 tablespoon), Dijon Mustard (1 teaspoon), 2 tablespoons Extra-Virgin Olive Oil (to taste), 1/4 teaspoon salt and 1/4 teaspoon pepper. Set aside.
Heat a grill pan over medium-high heat. Remove the chicken breasts from the bag, pat dry with paper towels, and grill for 3-5 minutes on each side, until there are dark grill marks on each side and the breasts are just cooked through.
Add the vinaigrette to the salad and toss to combine. Add the Blue Cheese (2/3 cup) and toss gently. To serve, place each chicken breast on a plate and top with a mound of salad. Serve immediately.