Puree basil, 1/4 cup mayonnaise, lemon juice, and mustard in processor until smooth. Transfer to small bowl; mix in remaining 1 cup mayonnaise and season to taste with salt and pepper. (Can be made 1 day ahead. Cover and refrigerate.)

Preheat oven to 350°F. Arrange pancetta in single layer on rimmed baking sheet. Cook until pancetta is just crisp, about 15 minutes. Transfer to paper towels to drain. Reduce oven temperature to 200°F.

Sprinkle all green tomato slices with salt and pepper. Place 1/2 cup flour in shallow bowl. Mix remaining 1/2 cup flour with cornmeal in another shallow bowl to blend. Working with 1 green tomato slice at a time, coat with flour, then egg, then flour-cornmeal mixture. Transfer to baking sheet. Line second baking sheet with paper towels. Heat oil in heavy large skillet over medium-high heat. Working in batches, fry green tomatoes until golden brown, about 2 minutes per side. Using slotted spoon, transfer to prepared baking sheet; sprinkle with salt and pepper. Keep warm in oven.

Sprinkle red and yellow tomatoes with salt and pepper. Place 1 fried green tomato slice on each of 6 plates. Spread each with 1 teaspoon basil mayonnaise, then top with 2 tablespoons arugula, 1 slice pancetta, 1 slice red tomato, another fried green tomato, 1 slice yellow tomato, pancetta, and another 2 tablespoons arugula. Spoon 1 teaspoon mayonnaise over arugula, then top with remaining fried green tomatoes. Spoon 1 teaspoon mayonnaise atop green tomato. Finish with remaining pancetta and serve.