To Make the Fried Tortilla Strips: In a deep, heavy frying pan, heat several inches of vegetable oil to a temperature of 375 degrees. Working in batches if necessary to prevent overcrowding, carefully add the tortilla strips to the hot oil, submerging them with a metal skimmer or slotted spoon. Fry the tortilla strips until evenly golden, 1 to 2 minutes. Carefully lift them out with the skimmer or slotted spoon and transfer them to paper towels to drain and cool. Set aside, uncovered.
To Make the Garden Herb Ranch Dressing: In a mixing bowl, use a fork to stir together the mustard and cold water, forming a paste. Set aside for 10 minutes. Add the remaining dressing ingredients to the bowl and, using a handheld electric mixer at low speed or a whisk, blend together just until smooth, taking care not to incorporate too much air into the dressing. Cover with plastic wrap and refrigerate.
To Make the Grilled Garlic and BBQ Chicken: Preheat a stovetop grill or the broiler. In a mixing bowl, stir together the olive oil, garlic, soy sauce, and salt. Turn the chicken breasts in this marinade and leave to marinate at room temperature for about 15 minutes. Grill or broil the chicken breasts until cooked through, 5 to 6 minutes per side. Chill thoroughly in the refrigerator. Cut the chicken breasts into 3/4-inch cubes and, in a bowl, toss with the barbecue sauce to coat well. Cover with plastic wrap and refrigerate.
To Make the Salad: In a large mixing bowl, toss together the lettuces, basil, jicama, Monterey Jack cheese, beans, corn, cilantro, dressing, and half of the Fried Tortilla Strips. Transfer the salads to chilled serving plates. Surround each salad with diced tomatoes and the remaining Fried Tortilla Strips. Top each salad with chunks of Grilled Garlic BBQ Chicken and drizzle the chicken with the barbecue sauce. Garnish with the scallion.